Stand four boards that are 1-by-1-by-16 inch on their small ends to be the chair legs. Set a 1-by-18-by-18-inch board on top of the legs to be the seat. Straighten the chair legs so these are in the corners of the seat board, with their sides even with the sides of the seat board. Drill two pilot holes down through the seat board and into each chair leg. Use 3-inch wood screws to fasten the boards together.
Position a 1/2-by-18-by-21-inch board, which will be the backrest, against the rear of the seat board so the bottom edge is even with the bottom edge of the seat board. Predrill four evenly spaced pilot holes through the backrest and into the seat board. Drill 3-inch wood screws through the pilot holes to join the boards together.
Arrange a 1-by-17-by-21-inch foam pad on the backrest so the edges of the foam line up with the edges of the backrest. Attach the foam to the board with six evenly spaced staples through the foam and into the board. Arrange a 1-by-18-by-18-inch foam pad on top of the seat so the edges of the pad are even with the edges of the seat. Attach the foam pad to the seat with four evenly spaced staples.
Drape fabric over the top of the chair and stretch it down tightly over the backrest and the seat. Wrap it over the sides and staple it underneath the chair about two inches from the edge. Space the staples every four inches around the base of the chair, stretching the fabric tightly before each staple. Cut off all excess fabric with scissors.