Felting slippers can be made in a number of ways, using a resist of some sort. Some people use plastic templates and felt flat, while other prefer to felt over a 3 d form… either a foot form (usually polystyrene), or a boot.
Here are a couple of on-line tutorials I have found that might help you;
This is a great little tutorial from a good felt maker.
Next is a video on You Tube;
Watching this will give you a basic understanding of the process.
Books are also a great resource for this topic. Here is one I recommend;
The shoes in here are baby booties, but the principle is the same for adult show making.