Measure the perimeter of your chair. Write dimensions of your chair on a piece of paper. Add a 5/8-inch around, for the seam-allowance. Determine the desired depth of your chair cushion. Divide the depth of the cushion in half and add that amount to the length and width of your fabric. Make a paper pattern using the measurements. Take the original dimensions for the chair and make a separate pattern for the foam.
Cut fabric using pattern as a guide. Lay fabric right sides facing together on a table, pin to hold in place. Thread your sewing machine. Sew fabric along 5/8-inch seam-allowance. Leave a section open for the foam. Turn right sides out, and press with a iron.
Lay the pattern over the foam. Use a marker and trace shape on to the foam. Cut the foam with scissors or a electric carving knife. Stuff foam into the fabric cover and sew the opening closed.