I started with 14 gauge mild steel, cut it into the desired shape and deburred the edges. (see also: Building Spaulders- An exercise in basic hammerwork, finishing, and assembly) I marked the center line of where I want the apex of the knee with a permanent felt tipped marker.
A slight shape is dished into the piece by striking it with the rounded face of a small sledge. Only around 15 hits over the dished wooden stump are needed to give it a beginning shape. These should be pretty hard hits. The hand holding the steel may get hurt from the shock transfer of the strikes even if you wear a glove.
The dishing is just to give it a starting point. I don't suggest dishing beyond this point as any further may cause the center to stretch too much and become thin.
Don't worry if it isn't even or pretty. It will get a lot uglier in the process before it starts to look like a knee.