Measure the width of the seat, from the outer edges of the frame, using the yardstick or tape measure. Measure the length from the front of the frame's seat to the top of the backrest. Accommodate for the part of the seat that hangs down a little, where the seat and backrest meet. Six inches extra should be enough to cover that, but measure to be sure, and add 9 inches to the length for the casings. Do not add anything to the width because the sling should hang just inside the frame after the edges are finished.
Draw out the dimensions on the fabric, using the tailor's chalk. Make two fabric rectangles.
Cut out around the drawn lines, using the scissors.
Place the two rectangles with the wrong sides together. Fold the fabric over 1/2 inch all the way up each long side, then fold the edge a second time. The raw edges will be completely encased. Top-stitch 1/8 inch from the both the inner and outer edges of the fold. This will be thick.
Fold the top and bottom edges 1/2 inch over, then fold the edges again, making a 2-inch fold this time. Sew the large fold down with the sewing machine by top-stitching 1/8 inch from the folded edge you created when you made the 1/2 inch fold. Run a second line of top-stitching by sewing 1/8 inch inside the first line. This makes the casing you will slide through the PVC pipes at the top and bottom edges.