Remove the limp cushions and flatten them out, if necessary. Measure the length and width of the cushions with a tailor's tape measure. You will use the measurements to cut out pieces of material for the new cushions that measure 3 to 4 inches longer and wider than the measurements you took. If the cushion has a tapered or curved shape, trace the shapes of the cushions onto a piece of tissue paper to create a sewing pattern. Add 3 to 4 inches of material around the perimeter of the tracing to provide room for the sides and seams on the cushions.
Cut out a piece of 2- to 3-inch-thick foam to match the first tracing you made around the shape of the cushions to create a new padding insert for the replacement cushions. Cut two pieces of canvas fabric for each cushion section. Again, the fabric should measure 3 to 4 inches longer and wider than the foam pieces.
Lay the two pieces of material for the seat cushion together with the printed sides facing. Secure the fabric together around the edges with safety pins. Sew the material together on three sides creating a 1/4-inch seam. Repeat this process for the two pieces of material for the back cushion. Leave the bottom edge of the back cushion and the back edge of the seat cushion open to insert the foam pieces.
Turn the cushion covers inside-out and slide the foam cushion inserts into the fabric covers. You may have to work with the foam for a while to push the pieces into the corners and make the cushions look straight.
Attach zipper teeth to each side of the remaining openings of the cushion covers, so you can zip the remaining opening closed. This also allows you access to remove and replace the foam inserts later, as necessary.