the hell game newsposts are probably a better place, even if they're months/years old.
but anyway it's a little non-obvious. when you start a game, you start next to an obelisk called DEMO SHOP where you can buy bodyparts. at the bottom of that page, there's a button labeled 'EDIT BODY'. in there, clicking on a colored square and then on an unequipped bodypart slots that into your body at that location. some people on 4chan actually made a video of how to use the interface; it's really not my proudest moment as a game developer :V