Basic Gelatin Skin
Warning:For this technique, the Gelatine needs to be hot. When hot, gelatine is not unlike molten plastic. If it gets on your skin it will scald and burn without coming off. Needless to say, eye protection must be worn.
Start by preparing your gelatine, as per the instructions for a gelatine base.
Put some cooking oil onto a paper towel and apply very thinly to your plate / tray. Wipe off any excess. Place the plate in a warm oven (250 degrees), until it is warm (but not hot) to the touch. The reason I use a plate not a tray is that China retains its heat longer than metal. The reason we do not want the plate to be hot is that gelatine 'denatures' or becomes useless above a certain temperature.
Pour a small amount of liquid gelatin onto the warm plate. If you get any on your skin, immediately put the area under a cold tap. Tipping the plate, try to spread the gelatin as thinly as possible. The warm plate should help keep it liquid. I have had some success using a hairdryer to 'blow' the gelatin flat. I have also tried using the oven again with some (limited) success.
Once you are happy that the gelatin is as thin as you want, place the plate on ice to cool it quickly. Note: Do not just stick the plate in the freezer. The heat will melt any food it comes near, which is not a good idea.
When it is cooled, you should be able to peel the sheets off the plate, though care should be taken with the very thin edges, as these can be the most useful part for seamless application.
You can store sheets of gelatin between layers of greaseproof paper in the freezer. I have had some for 6 months that were good as new when I thawed them.
Sheets of gelatine can be used to create lengthy gashes, as you can easily manipulate them with a knife. They can be applied to the skin for temporary purposes just by (gently) heating with a warm hairdryer, though this technique will often not hold long. A thin layer of liquid latex will hold it happily in place for longer terms. Better still though is to use Spirit Gum. With this, you apply the gum to the area, and wait until it is tacky to the touch, then carefully place the gelatin in place. To remove this you will need spirit gum remover (or mineral oil/baby oil).
One of the nice aspects of working with sheets of gelatine is that you can use it to cover other things, allowing you to 'build' underneath before applying the 'skin'. So for example you could place maggots on the skin, then only stick the gelatine to the skin around the maggots. Then cut through the gelatin across the 'maggot' area, and you have a wound with maggots in. This would be more difficult with latex skin techniques. See below for a tip to improve this effect.
The technique above will give you transparent skin sheets. This is very useful for seamlessly applying onto skin, but if it is not directly over your skin (e.g. flaps, covering other things etc), you want it to be opaque, and preferably match your skin tone.
The best method I have found for this is using cheapo acrylic paint, which you can buy in myriad colors often in small $1 tubes. Literally a drop or two of this in your gelatin mixture will easily mix in, and you can create excellent realistic skin of any tone that can be peeled and more.