Ok, I'm not real sure how other folks do it, but I have a system that works for me every time (it has so far). I take and old dark t-shirt and cut a piece large enough to cover the entire seat with plenty of extra... Then I lay the t-shirt over the seat and begin to secure it on the bottom of the pan. I basically cover the seat with t-shirt but not so tight that it mishapes the t-shirt. By doing this, you create a pattern that is flexible enough to contour the seat, but will lay flat when you go to cut it...
I use dark t-shirts so my WHITE grease pencil can mark on them easily.
Once I have the t-shirt secured to the seat pan, I use a grease pencil to mark a seam (or the edge of the pan).
Once I've marked all my lines I remove the t-shirt from the seat, lay it out on the table and let it lay flat. Don't force it flat! Just work it until it lays down. Tape the edges to a piece of posterboard so that as you cut your pattern, the t-shirt doesn't shift and ruin the shape. Once you have it all taped down, take your rotary cutter and follow the grease pencil lines.
Now you have a perfectly shaped pattern to work with!
!! REMEMBER !!
This method is designed to capture the shape of the finished piece of leather. Don't forget about your seam allowances if you plan to sew. However, if you plan to LACE your seat, you end up with perfectly shaped pieces!
I hope this helps!