Fit right fasteners. They are pins that go over the snaps and allow you to pull the material tight and mark where you need to cut and pleat. A must have for any snap cover, be it a boat or a pick-up tonneau. Available from many sources. Rochford Supply carries them but I have found them cheaper.

I tend to use the old cover to give me a ballpark shape then kind of shape it on the seat.
Or if i don't have that i use calico or scrap vinyl and mockup it the use the mockup as a pattern.

I have used the Fit rights for a while, I just hate the hole that is left if you have to move something.

If you are using support poles allow for a little play in the material. Once you raise the poles the holes should be at about the level of where you bind the edge of the material. If you are using a product like Top Notch you can rub the weave and the holes go away. I have had holes in other materials that bother me. On some I take a pin and put a drop of super glue in the hole.