Stand up four 1 1/2-by-1 1/2-by-18-inch posts on their smallest ends and put a 1/2-by-14-by-14-inch board on top of them to be the support board. Drill a hole down through the support board and into each of the four boards, which are the legs of the chair. Secure the legs in place by tightening 2-inch wood screws in each hole with a drill.
Position a 1/2-by-1 1/2-by-11-inch board in between each pair of legs with its ends touching the legs and its long, thin side touching the bottom of the support board. Drill four holes down through the top of the support board and into each of these boards, which are the side boards. Fasten them in place with 2-inch wood screws.
Set a 1-by-16-by-16-inch board upside down on the ground and set the chair on top of it, with the support board touching this new board, which is the chair seat. Drill six holes through the support board and into the chair seat. Secure the boards together with 1-inch screws.
Set the chair right-side up and place a 1-by-16-by-16-inch piece of foam on top of it. Cover the chair seat with fabric and flip it over. Pull the fabric tight and staple it to the bottom of the support board, placing the staples every 2 inches next to each other, making sure the fabric is tight around the legs. Remove excess fabric with scissors.