Why not
: just skip a step and use the cement mold to make the seat directly?

I couldn't figure it out either so I put a layer of epoxy on the mold, waxed the mold well, covered it with saran wrap and used that. I put on three or four layers of glass, took it off green (soft) which was still difficult to do and put two more layers of glass on at the end. The mold is still in good enough shape to make more seats so I'm not sure why the extra steps.
good luck
don