Measure the area you want your cushion to fit. For instance, if making a pillow seat for a chair, measure the width and depth of the seating area you are covering. If measuring for a window seat, measure the area you wish to turn into a seating area.
Turn the fabric inside out and sew three sides of the cushion creating a 1/4-inch seam around the three sides. Ensure the stitches are consistent.
Turn the fabric right side out and slide the cushion into the opening of the fabric. Ensure the cushion is evenly distributed throughout the width and length of the fabric.
Hand stitch the end closed using a needle and sturdy thread. A zipper can be added to this side if you want the fabric to be removable for easier washing and replacing.
Place the cushion into the seating area to ensure a good fit. Try out the cushion by sitting to ensure the correct feel has been obtained.