This may seem like a silly question, but although I've been using booru-type image networks for a while now, I've never actually needed to try to tag something on existing pictures. There are a few out there at the moment with the character_request tags that I actually recognize the character/series/whatever, but despite my best efforts at wracking my brain I can't seem to figure out what button will make it go. I was hoping it might be in the "Tagging Guide" but that just tells you what to tag, not how D:. Any chance someone might be able to shed some light on this?

The button you're looking for is "Edit". It's underneath the pic.

Aha! I may feel sufficiently like an idiot now, but at least I feel sufficiently like an idiot that can now tag some of these images. Thanks.

Thanks for tagging! ^^

Also, thanks for mentioning that the tagging guide doesn't explain how to tag. I never thought of it. I'll go edit that now.

I have a question about the source field. I have noticed that when I put a URL into the source field it does not auto-convert into a hyperlink. However, I have noticed that on sites like Danbooru or Gelbooru, the URLs in the source field are almost always hyperlinks. Is there a Booru tagging protocol which we're to use? Some guesses include:
1) standard HTML, so like <a href="http://urlgoeshere">hyperlink text</a>
2) BB Code, so like [url="http://urlgoeshere"]hyperlink text[/url]
But I don't want to fool around experimenting with it just because every time you make a tag edit it shows up in the history and that bothers my OCD side. >.>; So I'd rather just ask. And maybe this question, and the answer to it, will help others avoid blindly experimenting too.

I just assumed it was another feature that we don't get as free users, since hyperlinks are automatically created in the forums and comments. The only way to find out for sure would be experimentation. Edits to the source field don't show up in the history (and if it bothers you that much, don't check the history :P).