Simple, go into the info window (Alt+3 in Winamp or right click and select View File Info), there you can fill in any of the info and save your changes.
For fields like year, genre, game, and maybe artist, which are the same between all tracks, you can use PSFPoint to set these tags on a bunch of files at once.
When you have finished tagging the files, all that has changed is the .miniusf files, so those are all you need to upload if you want to share the changes.