Leave it there for fifteen minutes or so. Trim off any surplus. Leave it somewhere to dry. This makes it possible to form pieces. You must work quickly, since the stretchiness goes away in a minute or two.
Elbow cops are easy.
When the piece comes out of the hot water, put it into one of the bowls, forcing it down with your fingers to stretch the leather into the bowl, then put in the other bowl, so that the leather is being stretched between the two. Let the leather dry. Details will vary according to the size of your elbow and knee and how thick your padding is going to be. There are four ways to try to make it. Use your arm, wrapped in a towel, as the mold.