Cut a piece of cardboard to use as a template of the seat.
Trim the mold-resistant foam to match the template.
Place the foam in the chair to check for fit.
Stretch out the uncut fabric on your work surface. The right side of the fabric needs to face you.
Place the foam in the middle of the fabric. The part of the foam that will be the front edge of the cushion needs to face the middle of the fabric. Bring up one side of the fabric. Three sides of the cushion should be open, but the front is closed.
Pin the three open sides of the fabric. Insert a pin every few inches, about 1/4 inch from the foam. Fold in fabric at the corners if needed for a smooth fit. You should be looking at the wrong side of the fabric. Trim off any excess fabric.
Sew the two sides shut. Now pull the fabric through the open backside, so you are once more looking at the right side of the fabric. Insert the foam cushion.
Peel the back off of the self-adhesive hook-and-loop tape. Attach one piece to each side of the opening in the back. Press the two parts of the tape firmly together.