17. Attach No. 1 it to the underside of the seat piece of MDF with glue and stainless-steel screws. It should be aligned so that all the holes in the two pieces are centered on one another, allowing for a ¼-inch overhang of the MDF on each end front-to-back, and ¾ inches overhang side-to-side, which overlaps the steel frame. Repeat with No. 3.

19. Insert a tennis ball into each hole. They are available used, quite cheap, on eBay. If you have done everything right so far, all of the balls should fit snugly and none should push through.

20. Fit No. 2 over the balls in the seat pan. Again, everything should fit snugly, trapping the balls inbetween two retaining edges created by the plywood. Glue and screw into place. Repeat with No. 4 for the face of the back.

22. Now use twelve 1/8-inch diameter machine screws with washers to go through the holes drilled in Step 15 from top-to-bottom through the frame and both pieces of plywood. Tighten nuts and washers on the underside, snug against the frame. Use a Dremel or angle grinders to trim any excess screw length flush with the nut.

23. Cut four strips of plywood that are 3/4x161/4 inches. Sand, stain, and seal as desired. Glue to the four remaining exposed edges of MDF.

24. Sit down and take a well-deserved rest!