Remove the backing from one side of a piece of double-stick tape and place it 3 inches from the right side of the foam cushion.
Peel off the backing on the other side of the double-stick tape. Fold the right side of the foam cushion over and press it on the tape to hold it in place.
Set the cushion cover on a table or flat countertop with the zipper near the edge of the surface. Unzip the cushion zipper.
Hold the cushion next to the cover opening with the right taped edge pointing up.
Slide the left side of the cushion into the cover, pushing it toward the back until it touches the back of the cover. Turn the inserted foam to the left and work it into the left side of the cushion while pushing the right side in the cover to the right.
Position the foam cushion against the left side and smooth it out between the cushion top and the cover with one hand.
Hold the left hand on top of the cover to hold the cushion in place and slide the right hand into the cover. Pull the folded foam off the double-stick tape and smooth the right side of the foam against the right side of the cover.
Zip the cushion cover closed and smooth any wrinkles out with both hands.