Rotten Zombie Flesh Prosthetic
This prosthetic can be prepared in advance (a few days if you keep it in the fridge or freezer). Foaming the gelatine gives a better surface to apply makeup to, and makes the prosthetic lighter for easier application and longer stick. With a little practice you can make prosthetics that look better than store-bought, and the beauty of it is you can customize them exactly to your needs.
This is a very basic use of the gelatine as it requires no moulds, and the 'sculpting' is rather slapdash. It is a good first start to test out the materials.
Preparing the gelatine
Take your gelatine base, cut a piece about 2 inches cubed, and place in a microwave safe bowl.
Follow the instructions here for how to make foamed gelatine, at which point you should have a small bowl of (hot) foamed gelatine.
You will need to work extremely quickly at this point as the gelatine will be cooling rapidly and it is hard to 'refoam' it in the middle.
Making the prosthetic
Warm a plate gently so that you can still pick it up, and pour/scoop on some of your foamed gelatine. Using a spatula or flat knife, smooth it into a flattish but textured piece the size you want your prosthetic, and try to ensure that the edges are extremely thin, to blend with your skin (the warm plate should help this)
At this point your bowl of foamed gelatine will be cooling rapidly. Using the handle of a spoon, or a chopstick (my preferred implement), get some gelatine on it and pull it across your flat piece to make raised areas and stretched stringy segments. You may also find that tilting your plate vertically helps to get a drippy effect.
If your gelatine is cooling too quickly you can give it very short (3 second) bursts in the microwave to loosed it up, though it will lose some of its foaminess in the process.
When you have finished, and the plate is cooled to room temperature, place the plate in the freezer to chill the prosthetic. This will make removing it much easier and less likely to break.
After about 20 minutes you can remove the plate from the freezer. Very carefully, taking care with the fragile edges which are necessary for a convincing blend, peel the prosthetic off the plate. You will end up with a rather rubbery piece of gelatine. Trim any sections that need it. At this point you may store the item in the fridge or freezer between sheets of greaseproof paper if you don't need it immediately.
You can apply it using either spirit gum or liquid latex. Spirit gum will have a much better hold, and is less temperature sensitive than latex. It is also less likely to cause allergies, but is more expensive and harder to source, so latex will work fine.
Latex is also useful for sticking down stray edges.
Whichever you are using, the principle is the same. Quickly paint the entire back of the prosthetic. If using spirit gum you should wait until it is tacky to the touch before applying.
Ensuring the area of skin is relaxed, apply the wound pressing in the centre first then smoothing outwards. If any edges are not stuck down you can either paint under them with the latex/gum or paint over the top with the latex. Be aware that painting over the top may cause issues at the makeup stage, as there will be three surfaces (skin, latex, gelatine) which you will be trying to blend which is considerably harder.
As with any effect, your primary concern is blending the areas where there is a join between skin and effect. If you have color matched your gelatine well this should be easier, and if the foaming was not too much the texture may even be similar. Hopefully just some judicious use of cream base will blend it, but often some hiding work with greasepaint will be necessary. Trying to match areas in the middle of the flesh with areas of actual skin elsewhere is a good idea. Echo similar themes across the face.
With this particular effect an oozing over-the-top look is effective anyway, so concentrate on greeny yellow colors first. Stipple the lighter colors first around the joins, and if there are still obvious links you can cover them with some darker stippling. The last resort of the badly joined prosthetic is blood. If the makeup is really not working, then just squirt a load of blood on at the last, and people won't notice anyway.
For the rotten look you will want to darken the shadowed and sunken areas. Dark greaspaint on a q-tip is effective for getting in the corners, greens and yellows for highlights. Be careful with the greasepaint as it will smear easily, and you can end up with everything being brown. Stippling (dabbing) is the best technique.
Finish the look of with some trickles of green ooze and red blood coming for the more torn looking areas.