Measure the width and depth of the window seat.
Cut a piece of ¼-inch-thick plywood that matches the measurements you took in Step 1, using a saw.
Cut a piece of 4-inch-thick foam that matches the measurements you took in Step 1, using an electric carving knife. This type of foam is available from the fabric store.
Glue the foam flat to the plywood using wood glue, lining up the edges together. Allow the glue to cure completely according to the directions on the package.
Add 6 inches to both the width and the depth measurements of the window seat.
Cut a rectangle of fabric that matches the measurements you took in Step 4.
Lay the fabric, pattern side down, on a flat surface.
Center the foam on top of the fabric, and the plywood on top of the foam.
Wrap the edges of the fabric up over the plywood and staple them in place with a staple gun. Your window seat cushion is now ready to place on the window seat.