If you've looked at the story list at any point since the last overhaul,  you've probably noticed that tags and summaries are not especially common for any entries. Even more recent stories can be hit-or-miss about having those fields filled in. Whilst it's possible to search stories by title or author, that requires knowledge of the thing you're searching for to begin with. Should you just be looking more generally, your only recourse is to either look at random or cast out into the darkness in hopes of recommendations, something not too common either on THP these days.  The only hope of fixing this rather deplorable situation is getting more people involved in tagging, which is what I hope to encourage with this post. First, though, I'd like to explain a little bit about what tagging affects on the site.
Something that came with the movement of the greater community to Discord was the installment of a webhook to announce updates on the site. Rather than making it the job of the authors to inform others that they updated, as was the case in the old days, a helpful hakutaku  raises her voice in the relevant channel to get everyone's attention. Even though anyone who might be interested ought to be checking the site out actively anyway, it has to be admitted that automated announcements can be a good reminder to go read. If you look closely at the announcements made by the webhook, you'll see that part of the included information is a story summary if it's available. Whilst not the most likely to be the first point of contact with a story, there is the opportunity there to be acquainted with the premise of an unfamiliar story.
This was mentioned in a recent Patreon post,  but part of the reorganisation of things on the site has to do with making it easier for search engines to find and index us. Part of that includes what information is served up to search engine spiders in brief, digestible form. Normally, that would just be a short extract of the page's contents. For stories with properly filled out metadata, on the other hand, the summary, author, and title will be served up in search results. That means that not only can people searching the story list more easily find stories of interest, people who aren't even on the site yet can too! This sort of overlaps with the next point, but it's fairly significant for the site, considering we're always in need of more reach to new audiences.
In the best of times, people browsing the web are lazy. You have to practically ram things down their throat before they notice what you want them to notice. This is unfortunate side-effect of the terrible overcommercialisation and general misuse of the web as an information channel. People carry their browsing habits over to THP, too. This means that it can be a challenge at times to get anyone to notice your story if you're a writer. It's a tale as old as THP.
The bad old days were even worse. We didn't have the Recent Updates box  on the front page, and the Story List was manually updated, so the onus was on the reader to comb through the boards to find what was active and of interest. With the way most people browse the web, this meant that anything that wasn't right up front ended up ignored a lot of the time. Things have become much better in that respect — for current stories.
What about old stories? Well, that's the real trick at this point. The primary way anyone seemed to discover stories of old before was through recommendations, either on-site or off-site, or just by digging through the old, crusty Story List. For most, the latter would be an exercise in tedium and frustration. This is where improvements can still be made. People in the Web 3.0 days expect tagging and precise targeting for everything when it comes to seeking out content. Without a degree of that applied to older stories whose writers have long abandoned THP or that are simply finished a while ago, they're likely to languish in the Story List and be forgotten. We're talking about THP history in a way, and I personally think preserving history is an important thing.
That's why I'm putting out this call for more anons to join in helping the tagging effort. As you can see from the reasons above, filling out all of the metadata for stories is important to the site in various ways. However, I understand that it's not the clearest undertaking in the world. Fear not, because I will do my best to explain the process and hopefully get you shoved in the right direction. You can look over this guide for general information beforehand if you want, but I'm writing this from the perspective of someone simply jumping in, so it may not make as much sense if you don't do that. I recommend that you do, as it's not terribly hard once you get the hang of it.
For starters, you need the right permissions to be able to edit Story List entries. Normally, writers have access to their own stories for tagging and summary editing, but there exist classes of users who have further access. Since it's not likely that you're in the running to be a janitor or higher, you'll be aiming for a tagger role, which will have to be assigned manually by the admin. The process for getting started is straightforward: contact Teruyo  to get a username, sign in via the supplied link, and change your password. From there, you're ready to get started tagging.
Now that you're hopefully logged in to the management page, if you look to the left, you'll see links under the header 'Story Management'. Of chief interest is the one currently labelled 'Stories missing tags'. This is the list of stories that have yet to have their information fully filled in. Much of the list is, as described, totally and utterly without tags or summary. Some do have tags, but they may be incomplete by the criteria that will be described shortly, and most also don't have a summary. Others may have the requisite information but only in stub form or otherwise not in an quality that would be of great use. Whatever the case, these are the stories that should be given priority. It is a formidable list at the time of this post, which is why we need all the help we can get.
Once you find an interesting entry, click the Modify button to be taken into the details. The first part of the information will all be things that you won't be able to alter as a tagger, namely the title, author, associated tripcode, and story status. Do note that this box has a link to the first thread of the story at the bottom. Take advantage of this to jump into the story when looking for information to tag. After that comes two boxes with the info that you can edit.
First up is the Tags box. This one is a fairly simple one, consisting of an admonition to include at least one character tag, one technical tag, and one genre tag, along with a text field to enter said tags and a link to a list of all current tags. Before starting on tagging, it would be advisable to take a look at the tag list and familiarise yourself with what's available to you. There is autocompletion to help you as you type in the text field if you can't remember a particular tag.
Now, this is where I'll explain some of the tagging criteria. Mind that none of this is set in stone as far as rules go; if there is any determination in the future by the community at large or by Teruyo, follow that instead. If you've seen the notice and the tag list, you'll notice that tags are divided into three types: character, technical, and genre.
Character tags should be fairly self-explanatory, but I will take the time to make a recommendation. Namely, whilst there is no set approach to tagging characters as yet, what I would suggest is to consider presence when adding character tags. To explain by example, let's consider a story that is three threads long. This hypothetical story features Marisa as a prominent character, so she appears in the majority of updates. On the other hand, Patchouli only appears in a couple of updates and doesn't have a big role in anything plotwise. Between these two magicians, who would be most natural to tag? The answer is that Marisa would be tagged whilst Patchouli would not. To slightly complicate things, let's add Alice into the picture. Alice also only appears in part of the story, but those appearances constitute more than a small handful of updates and she motivates part of the plot. Accordingly, we would say that she has enough presence in the story to warrant tagging along with Marisa.  Whew. That's a bit of an explanation, so I hope I conveyed something helpful. Let's move on to the category.
Technical tags are largely dedicated to more 'meta' elements of a story. That is, anything to do with the story from the standpoint of its formatting and any such relevant matter. Of biggest note here is the set of tags for denoting what the narrative perspective of the story is. If nothing else, letting potential readers know if the story is told in the first-, second-,  or third-person should be an early concern of tagging. Also of note are the tags for the protagonist(s). A criteria used by some to filter stories is, for example, if the story has a canon Touhou character as the perspective character versus an original character. Some people don't want to read another story about a human outsider. Others might want exactly that. These tags are for making that clear to their potential audiences. Look over tags like those and others relating to formatting  carefully, and be sure to add all that apply, taking into account the advice above about presence; if something applies to the majority of the story, tag it.
Third and last of the three categories, genre tags are a tad more straightforward than the preceding ones. Simply put, these are to tell potential readers what the story is 'about' in a rough sense. Applying the correct tags here is something of a matter of judgement and savvy. The best approach is to think about what someone looking at that tag might expect the story to feature primarily. For instance, is most of the story time spent on a budding courtship between Sanae and a high-class tengu? Sounds like a candidate for the Romance tag to me. An unsolved murder in the village? Mystery fits the bill. Of course, that doesn't mean that multiple genres can't apply to the same story. That said, try to limit genre tags to two or three maximum as a rule of thumb. Think hard about what characterises the majority of the plot and stick to what fits closest, even if it's not all-encompassing. When in doubt, try asking others and comparing viewpoints.
You might think that tagging is challenging enough, given some of the ambiguities inherent to the process, but now we get to meet the real final boss of story metadata: the summary. This is where a casual acquaintance with a story will not help at all; you really ought to have read the entire story and have a good grasp of what happens before writing a summary. Beyond that, writing a decent summary is more an art than a science, so it's hard to give good guidelines for it. Still, if I had to say one thing, it would be that you should aim for a generalised statement of the story's purpose over a synopsis. This can be challenging, especially when you're dealing with stories that died quickly or are written in a purposefully obfuscatory way, so even that's not a hard and fast rule. Basically, try to consider someone who's already read the story. Would they recognise the story they read simply from the distilled version you portray in your summary? Additionally, it's important to also consider how the story might come off to a new reader. You are, in essence, in charge of the 'sales pitch'.
That's about all the guidance I can give as to how to tag stories. One thing I'll say in conclusion is that this advice also applies to writers dealing with their own stories just as much as taggers helping to clear the backlog. Should you ever write your own story, it will come in handy to know how to deal with its entry in the Story List. Either way, by helping us keep this information filled in and correct, you're ultimately helping THP as a community. If nothing else, you're making it easy to find a story you enjoyed again if you feel like revisiting it later. Please consider helping out with tagging if you're not already doing so.
You'll have my personal thanks if you do. I seriously cannot do it all by myself.
 If you've somehow been around a while and haven't ever looked at the story list, shame on you. The link is right there on the front page. Go click it right now!  I'll probably do a post on this at some point.  Miss Keine~  I really ought to do a post on that in the nearest future; few people seem all that aware of the Patreon despite it being shoved pretty close to their faces.  Better known as the Mystery Box to those of us old enough to remember.  Via Discord is (unfortunately) the fastest way at the moment. If you don't want to use Discord, I'm not sure what your options are at this point beyond finding the feedback thread in /gensokyo/, posting there, and waiting patiently. With any luck, we'll have alternate means in the future.  As an aside, note that all character tags are given in the proper surname-first order. If you can't find the right tag for someone, try typing their surname first; i.e., 'Kirisame Marisa' instead of 'Marisa Kirisame'.  Although common in the early days, considering it was a staple of the old Choose Your Own Adventure books, second-person narration isn't all that common these days and is even met with a degree of distaste by some. If you're considering writing, it's recommended against now.  For those of the /tg/-and-adjacent milieu, the Quest tag might be a little puzzling. How does one differentiate between a Quest and a CYOA? The answer is a little bit complicated since there is, once again, no solid rule of thumb, but a distinction I think could be made is of the former being focused on roleplaying, kind of as a pseudo-tabletop session, as opposed to what would simply be a narrative with occasional choices. Of course, that's somewhat arbitrary. After all, I wouldn't personally call early THP stories 'quests', but they do share some similar hallmarks. In general, I would say to not use the Quest tag unless you can really make the case that it applies.