Cut a piece of foam to size using a utility knife or foam cutter. You can use an already made seat cushion as a template, or measure your seat with a measuring tape. An easy way to make your cushion exactly match your seat is to position the seat upside down on top of the foam. Trace around the seat with a marker and cut out the foam along the traced lines.
Position the foam over a piece of fabric, print side up. Fold the fabric over the top of the foam and trace along the sides. Cut off any excess material, leaving approximately 1 inch on all sides for seam allowance. Remove your foam from the fabric. Pin the sides together along your traced lines.
Sew the two opposite sides together to make a pocket, using a straight sewing machine stitch. Add an extra line of stitching on both sides of your pocket for extra strength in your cushion. This will prevent the threads from ripping when you insert your foam. Cut off excess material around your seams. Turn the fabric right side out.
Heat your iron to the proper setting for your fabric. Press the seams to remove any wrinkles. Slide your cushion foam into the fabric pocket, pushing it firmly into the corners to ensure a proper fit. Sew the open side closed by hand.
Sew 3-inch pieces of ribbon to both ends of the back of your kitchen chair cushion. Attach your cushion to the chair seat, using the ties to secure the cushion to the back corners of the chair. To hold your cushion securely in place, add a hook-and-loop fastener set to the bottom of the cushion and to the chair's seat.