Measure your window seat area. Decide whether you want a flat cushion or a sculpted one. Memory foam comes in wedges, flats, waved and egg-crate styles.
Cut a piece of memory foam to the dimensions of your window seat.
Lay foam piece on your chosen fabric. Pull fabric over foam as if you were about to wrap a package. Use a yardstick or other long, straight edge to keep lines straight while cutting your fabric.
Use seam binding on the long edges of the fabric. Fold the short ends package-style over the memory foam and tack in place with needle and thread.
Stitch plastic hook and loop tape strips along the long edges of your window seat cover. This will enable you to remove and clean the cover as needed.
Put your window seat cushion in its place.