Lay a sheet of plywood flat so the 21-inch edge is horizontal to your body. Measure 4 inches away from the top right corner, along the top 21-inch edge and draw a 24-inch line that starts at the edge and is perpendicular to the edge. Draw another line that is perpendicular to the first, that starts at the bottom end of the line and ends at the left, 48-inch edge of the plywood. Repeat this step with the other sheet of plywood. You will have a 17-by-24-inch rectangle drawn on each sheet.
Cut along the lines using your circular saw on both sheets of plywood. These are the sides to your wooden booth.
Set the sides on their 48-inch edges so they're parallel, so the cuts are aligned and so they're spaced 40 inches apart. Lay six 40-inch boards between the sides perpendicularly so the boards are flat, against each other, and so one of the boards is flush with the ends of the plywood. Drill 3/32-inch holes through the sides and into the boards so two holes enter each board at each end. Drill 1/4-deep countersink holes in each of the 3/32-inch pilot holes and screw the sides to the boards. This is the backrest to your wooden booth.
Position the last five 40-inch boards perpendicularly between the sides so their edges are against each other and so they're flush with the 23-inch vertical edge. Screw through the plywood and into the boards just as you did with the first set of boards, drilling pilot holes and countersink holes. These boards make up the seat to your booth.
Apply your wood putty to all the holes surrounding the screw heads on the booth. Let the putty dry and sand down the entire booth, first using medium-grade sandpaper and finishing with fine-grade sandpaper. Wipe the wooden booth clean using a tack cloth and apply wood stain. Let the booth dry before sitting in it.