pretty simple. If you want to ensure success, doing a chalk outline and using a template first would probably help, but I didn't and I don't mind the slight messiness of mine. To add lightness and create the illusion of depth I then used two parts water to one part bleach to go over the fronts of the ribs/etc.
Lay your shirt out flat, put some magazines/newspapers inside. I like using magazines, but if you use newspaper, make sure you use at least 5-10 pages so the bleach doesn't bleed through.
start painting a first coat of the entire design with four parts water to one part bleach. I freehanded mine, because I'm pretty partial to drawing skeletons. but I'm sure you could find a pattern online to follow if you so desire.
go over the fronts of the ribs/sternum/whatever else should be in the foreground again with two parts water to one part bleach. I also added highlights to the vertebrae. Its important to let the bleach dry after this step, don't overdo it before you can tell how light its going to be.
bleach in any missing bits/any bits you'd like to be lighter after it dries (takes perhaps 15 minutes, don't worry)
when you're satisfied, give it a quick rinse and let it dry sans bleach before you try it on :]