Straight window seat cushions are available in retail stores, but bay windows pose a special dilemma as the space could be an uncommon shape or size. Finding the right cushion for a bay window seat is difficult unless you design it. A bay window seat cushion is not a complex project, so you can make several to change it out with the seasons.
Roll out the craft paper on the floor in front of the bay window. Cut the paper for the template, or pattern, slightly wider than the window seat.
Place the paper on the seat. If the seat is deeper than the width of the paper, attach another piece with tape.
Press your fingers into the corners and edges around the perimeter of the seat, moulding the paper to the seat's shape. Cut through the paper, following the outline of the seat.
Lay two sheets of poster paper on the floor, side by side. Tape the sheets together.
Center the craft paper template on top of the poster paper. Trace around the template, 2.5 cm (1 inch) wider than the edge of the craft paper, transferring the dimensions to the poster paper with a pencil. Cut out the poster paper template.
Fold the upholstery fabric in half along the grain, wrong side out.
Place the poster paper template on the fabric. Trace around the pattern with a fabric marker. Cut through both layers of fabric, following the marker line. Spread out the remaining fabric in a single layer. Place the pattern on top and mark around it. Cut the fabric at the outline mark.
Spread out the batting. Place the poster paper template on top of the batting. Cut the batting around the pattern.
Spread out one piece of cut fabric, wrong side up. Arrange the batting on top, matching the edges. Place another layer of fabric over the batting and match the edges. Pin the pieces together around the perimeter. Sew around the pinned edge with a 2,5 cm (1 inch) seam allowance. Remove the pins.
Turn the sewn cushion with the fabric right side up. Arrange the remaining fabric on top, wrong side up. Match the edges. Pin the layers together around the edge. Sew through all layers of fabric and batting with a 2.5 cm (1 inch) seam allowance. Leave an 20 cm (8 inch) section of the edge open. Cut off excess fabric close to the seam line at the corners.
Reach inside the cushion through the opening. Grasp the opposite end of the cushion inside and pull it through the opening, turning the cushion right side out. Reach back inside the cushion. Push out any bunched material around the seam, making the cushion lay smooth and without puckers.
Fold over each edge of the opening 2.5 cm (1 inch). Turn the edges toward the inside of the cushion. Pin the edges together. Sew a 0.6 cm (1/4 inch) seam along the fold and remove the pins.
Use more layers of batting for a thicker cushion. Add tufting with a special upholstery needle, upholstery thread and decorative buttons.
Search the fabric for pins before turning the cushion right side out.