Here is the window seat cushion I completed for a client. Not to difficult...the 100 inch sleeping bag zipper I had to use was the biggest zipper I've ever used but it worked perfectly!
First trace the space where you want to create a seat cushion. As you can see I used some butcher paper for my template. Then I pinned it down to the foam and cut the foam to size.
I then laid out the zipper to make sure it was going to fit or see if it needed to be cut down
cut your fabric to size
sew and stuff
ready for delivery!!!