Measure up 11 inches from one end of a 24-inch plank of 1-by-12 lumber and mark a line across. Glue a plank of 12-inch 1-by-2 lumber on the marked line, with the 2-inch face of the lumber glued to the piece of 1-by-12.
Secure the 1-by-2 plank to the 1-by-12 lumber with three evenly spaced wood screws driven through the 1-by-2 into the 1-by-12 lumber.
Place a second 12-inch piece of 1-by-2 along one 12-inch edge of a piece of 11-inch 1-by-12 lumber and glue it in place. Secure the 1-by-2 lumber with screws as you did for the first piece.
Stand both the 24-inch and 11-inch pieces of 1-by-12 lumber upright, so that the 1-by-2 planks attached to them are parallel. Move them apart 12 inches
Glue a piece of 12-inch 1-by-12 lumber, which will be a square, onto both the 1-by-2 planks so that a small “h” shape is formed. This is the seat of the chair, with the 24-inch plank the back, and the 11-inch plank the front.
Check that the corners formed by the seat and the back, and the seat and the front are all 90-degrees using a carpenter’s square before the glue sets.
Flip the chair over and place it seat-side down on a work bench, with the back of the chair over the edge of the work bench so the seat can lie flat.
Secure the seat to the 1-by-2 planks with three evenly spaced screws driven through the 1-by-2 planks into the underside of the seat.
Return the chair upright, then paint in the desired color or colors and let dry before using.