Remove the cover from the spa. Open the zipper and carefully remove the foam inserts. Shake out any loose debris and lay the cover upside down on a flat surface.
Carefully remove the zipper from the cover with a seam ripper and save for later use. Make a quick sketch of the cover, labeling each separate part of the cover, including the placement of straps and handles. Using masking tape and a pen, mark each section of the cover for reassembly. This will make reassembly much quicker and less frustrating.
Open each seam with the seam ripper and disassemble the spa cover completely. Save the mesh bottom cover for reuse.
Lay the new vinyl or canvas, right side up, on the ground or a large work surface. Place the individual pieces of the spa cover on top of the new material, right side up. These pieces are the template for the new cover. Tape the pieces to the new material with small pieces of masking tape to avoid movement when cutting.
Cut around each piece of the template with very sharp scissors. As each section is cut, remove the label from the old piece and place it on the corresponding replacement piece. Cut new straps and handles as well.
According to the sketch and labels, reassemble the cover using a sewing machine with a heavy duty needle and thread. Add straps and handles in the same places they were when original cover was taken apart. As each seam is sewn back together, tape a 4 inch wide strip of wax paper to the material. This will make the needle slick as it punches through the heavy material. The wax paper will also help the material slide better under the presser foot making it easier to keep the seams straight.
Reattach the mesh bottom and zipper to the new cover. Insert the foam pads and close the zipper. Waterproof all seams with roll on waterproofing according to the manufacturer's directions.