Lay all four of the 68-inch boards flat and together so the ends are flush. Lay a 24-inch board at each end of these boards. The ends of the 24-inch board need to be flush with the sides of the two outside 68-inch boards. Screw two 3-inch screws through the 24-inch board, into each 68-inch board at both ends.
Lay the 8-inch board flat and horizontal. Notch out a 4-inch long by 3-inch deep section of this board with your handsaw, chisel and hammer. It should be centered, 2 inches from each end of the board. This board will be one of your door handles. Stand the 8-inch board on its 8-inch edge so the notch is facing the ground. Drill a 3/8-inch hole that is 3 inches deep on the top 8-inch edge, so it is 1/2-inch away from the end of the board. Drill another hole in the same manner at the opposite end of the board, and repeat this process on the other 8-inch board.
Position your door handles on the door where you prefer them to be. Be sure to have the notch facing the door. Screw through the holes with the 4-inch screws and into the door. Sand down the door and wipe clean with your tack cloth.
Screw your half hinges to the door and the other half hinges to the door frame. Be sure the door is able to close tightly, and most importantly, open outward from the sauna.