To build an Adirondack chair, cut treated boards for the chair's side and back rails, legs, slats and arms. Attach the bottom back rail, side rails and slats for the seat; the top back rail, rear legs and slats for the back and screw the arms to the chair's legs.
The lumber boards for an Adirondack chair must be knot-free and straight. Cut the wood using a jigsaw, and sand it using 1/4 inch-bit router.
For the chair's seat, cut two 1-by-5 1/4-by-33 3/4-inch side rails and one 1-by-3 1/2-by-23 1/4- inch bottom back rail. Cut nine 3/4-by-2 1/4-by-24 1/4-inch seat slats and two 1-by-4 1/4-by-20 1/2-inch front legs. Screw the side and bottom back rails using one 1 5/8-inch decking screw at each end. Evenly space the seat slats and attach to the side rails. Screw the front legs to the seat.
To make the chair's back, cut seven 3/4-by-3 1/4-by-35 1/2-inch back slats, one 1-by-4 1/4/-by-23 1/2-inch top back rail and two 1-by-2 1/2-by-29-inch rear legs. The width of the back slats must be 3 1/4 inches at one end and 2 1/4 inches at the other. Screw the rear legs to the side rails and the top back rail to the top of the legs. Arrange the back slats in a fan shape, and screw in place.
Cut two 1-by-5 1/4-by 28-inch arms and two arm supports of suitable size. Screw the supports and arms to the chair's front and rear legs using 2-inch screws. Stain or paint the chair.