How to Make a Homemade Bay Window Seat Cushion

Measure the length and width of the window seat opening. Determine the depth of the cushion, generally 1 or 2 inches thick. Add 1 inch to each measurement to allow for seam allowances.

Sketch an image of the cushion on a piece of paper so you can visualize each measurement. There will be one top piece, one bottom piece and four depth (thickness) pieces.

Cut the four depth pieces using the adjusted measurement that includes a 1-inch seam allowance. There are two pieces using the depth and width measurement and two pieces using the depth and length measurement. Cut the bottom and top pieces of the cover using the length and width measurements.

Sew the four depth pieces together with 1/2-inch seams so there is a loop of fabric that alternates the short and long pieces. Press the seams open with an iron.

Pin one long edge of the depth fabric loop to the long edge of the bottom cover fabric with the right sides together, matching the corners. Pin one side of a lapped zipper that is the length of the width side of the cushion to the width of the bottom. Pin the opposite side of the zipper to the width section of the loop.

Sew the zipper in place using a zipper foot on your sewing machine. Sew the pinned length of cover with a 1/2-inch seam.

Pin the remaining two sides of the bottom piece of cover to the depth fabric loop, matching the corners. Sew the two pieces together with a 1/2-inch seam.

Pin the top piece of cushion fabric to the depth fabric loop with the right sides together and seams matching. Sew the two pieces together with a 1/2-inch seam.

Open the zipper and pull the fabric through so the right side facing out. Smooth the corners with your fingers or a pin.

Cut a piece of upholstery foam to the cover dimensions and slide it through the zipper gap. Close the zipper and place the cushion on the window seat.