Are you making this from scratch? You are not copying the old seat? The front should have about a 42" long straight rectangle with each end sewn to a piece shaped like the one in the picture. The left side piece will be a mirror image of the right side piece, or a pair. You will have to adjust the size of the part to fit your seat top. You can see how this shape will turn the corner on both sides of the seat. I hope I'm answering your question correctly, if not, keep asking.
No one lives forever, the trick is creating something that will.