I've tried everything I can think of, and I'm maybe missing something obvious, but the option to actually create outfit tags doesn't seem to exist.

click on the saved outfit you want to tag, and type the tag into the "search tags" bar. if the tag doesn't exist yet, you'll have the option to create the tag. click that, and it will append the tag to that outfit.

it does seem to be a bit finicky as to whether or not the tag system will then show you the tags you've already made; a couple of the times i tried it, the tags i had made appeared as options to simply click to append to other outfits. most of the time, though, they don't show up at all and you have to retype them to add them to other outfits.