I watched a professional seat cover maker prepare a pattern for my Yamaha XJR with it's non-standard seat.
He worked with a big sheet of clear plastic, pinning it on and around the bare seat. He folded and marked the pleats in the plastic with a texta, and did the same on the plastic around the seat edge. He cut out the pattern and traced around the edges onto the back of a sheepskin. It was a prefect fit.
He made it look VERY easy.