Heavy Duty Staples & Staple Gun
After you have measured everything, (length & width) you can purchase your fabric, foam & plywood.
I measured that I needed 3 more inches of foam to make my cushion fit snuggly into the window seat. (I wish I could have had the plywood cut perfectly to fit but I don't own a chainsaw so i had to have a guy at Home Depot cut it for me.)
After I cut the foam to fit (the corners that I cut arent pictured in this photo i guess..sowwy)
I laid the fabric underneath the foam/plywood, folded it over, pulled it taut and began to staple. I stapled it every 2 inches or so.
Fold over the second half and staple again. Since my window is shaped differently than just a plain rectangle...and I had cut the foam to fit I needed to fit the fabric to that shape as well. i gathered and cut the fabric and then stapled it. Not the prettiest thing ever, but no one will see the bottom of the cushion.