Take Note: this tutorial is not necessarily the "right way" to do things, nor the "best way", but is simply how I went about it when I made my own. Anyways, on to the project!
I made my Splicer costume to join Chibik3r0's big group, but I didn't like how boring my skin looked, especially compared to the concept art I was using for reference. So after looking at my options, I decided that using gelatin to build up the tumors/mutations would be both the cheapest and easiest way for me to pull it off. Doing the makeup like I did is actually really simple, but it can take a long time to do since the gelatin needs time to set... it took me about 5 hours to do my arm, leg, and chest, but I'm sure that some of that was just inexperience. I based the technique on what I've seen friends do with liquid latex and cotton or tissue-based fillers.
plain gelatin powder (bulk food store)
glycerin (drug store)
cotton balls (drug store or dollar store)
powder foundation (drug store or dollar store)
snazzaroo makeup (costume shop or art store)
fake blood (costume shop)
microwave and microwavable dish
I made my initial mix of gelatin with 3 parts plain gelatin powder to 3 parts water to 1 part glycerin, though I might have changed the ratios a little between that test run and the final makeup, since I didn't take notes specifically regarding the mix. I know I made it on the runny side so it would smooth over the cotton balls easily. There are lots of recipes available online - basically you mix the ingredients and microwave it to get a smooth consistency and apply it while its warm (but not burning hot!!!)
Aside from making sure that the entire surface was covered (no holes or cotton visible), I let the gelatin go where it wanted, defining all the lumps of my cotton filler. Once the gelatin was set, I dusted the whole surface with a cheap mineral powder foundation (because it's what I had on hand). I don't have any photos of the powdered surface without the other makeup, but you can most easily see on my chest how it took the shine off and toned down the natural yellow-ness of the gelatin.
If you haven't worked with plain powdered gelatin before, it smells awful. Kind of like wet dog. The smell fades as the gelatin cools and sets, but it never actually goes away. (good to be forewarned about if you're using it on your face). It's also very cool to the touch once set - downright cold even, which was something that really surprised me. But there's some good news - the squishiness of the cotton balls under the makeup is pretty fun. I was welcoming people to poke my arm all night and enjoying the "EWWW" reactions XD