Drill twenty-four 2-inch deep, 3/4-inch diameter holes into each 1 1/2-inch wide edge of each 48-inch board. Space the holes 2 inches apart and position the center of the holes ¾ inch away form the sides of the board.
Glue the ends of each dowel into the boards so you have four pairs of boards, each with twenty-four dowels glued into them. Let the glue dry according to the instructions on the bottle. These are the sides to the playpen.
Position a pair of sides upright, parallel and 48 inches apart. Set the other pair of sides between them at right angles and spaced 45 inches apart. Screw through the 48-inch boards on the first pair of sides and into the boards on the second pair. Use two screws for each end of each 48-inch board.
Sand the playpen using medium-grade and then fine-grade sand paper. This will prevent any splinters from poking into your child's hands. You can also paint or varnish the playpen. Use only a non-toxic finish and allow the surface to dry completely before allowing the child inside.