Here's a short/long bit of info for you from my personal experiences:
There are several methods of doing gelatin burns. I've experimented different ways and done as much as 3/4 of my body in it before. The classic method is to smear the gelatin on the skin, coat/seal appropriately, then color from there. I will often do a textured latex underbase colored, then powder heavily and apply slightly thinner gelatin over top of that (then seal appropriately from there or do another textured latex layer over). Sometimes you get some nice separations where you can rip the flesh off through the acting night.
First mixing the gelatin:
Do 1:1 Gelatin to hot water (hot as you can stand on your skin and will not burn yourself) and coloring (coloring included in water, you can use food coloring, acrylic paint, liquid makeup, etc...) I usually tint it red or pink to simulate the under raw flesh look. Depends if you want fresh or healed burn effect. I have used liquid foundation mixed in for a decent flesh color. Some people dislike the smell of gelatin. One thing you can do to mask that is mixing in just a little bit of baby powder into the mix. It will give it a slight cloudiness so if you want it 100% clear I dont recommend it.
Work fast as it can set up pretty quickly!
Once you have applied it everywhere you want you can then coat it in either latex or castor oil (this is much cheaper from the drug store found near laxatives, but sold by makeup manufacturers for much more) Do not coat anything in latex that is going over your hair (including facial hair) there is a good chance it will not come out without losing some. However gelatin is safe to put in your hair (including facial hair) as long as you seal it with castor oil. A nice hot shower or dunk in a basin sink will melt it right out (just make sure to catch the clumps before they go down the drain. Make sure you coat it right away though as it will continue to dry otherwise and get very crunchy to the point where it could cut you...had it happen the first time I tried it on my own.
I have found if doing a crispy burn makeup, reds, yellows, pinks, and maybe some flesh and gray greaspaints lightly dusted then lightly brushed with black poweder turns out very nice.
If you want the more fleshy look color the gelatin and/or use flesh latex over and accent you ridges and pits in the gelatin texture to accentuate.
Then if you want to do the method I spoke of, do your latex texture with cotton or TP underneath, color with greasepaints, powder with baby or translucent powder, then apply a little thinner mixture of gelatin (maybe slightly tinted) over top of with a spatula or some kind of soft spreading tool.
Hope this info helps...Ive done a LOT of gelatin makeup jobs...so if you have any other questions feel free to ask!
Mike "Pogo" Hach