Design the bench around a custom cushion or have one tailored to fit. Contact a local fabric store or design studio for durable outdoor fabrics. Cut all redwood pieces precisely to your specifications for a seamless build.
Secure the 2" x 6" front brackets to the outside faces of the 4" x 4" front posts, 12 inches from the ground, using 3-inch stainless steel screws. Repeat for the rear seat brackets on the 4" x 4" rear posts.
Raise the front posts and front seat support assembly. Then position the rear posts and rear seat support assembly about three feet apart to form the bench depth.
Attach the 2" x 6" x 3' side seat pieces to the outside of the 4x4 posts. The frame is now complete: four 4" x 4" corner posts linked by 2" x 6" seat supports, positioned 12 inches from the ground.
Fasten two 2" x 4" seat slat brackets to the inside of the front and rear seat supports, recessed 1-1/2 inches so the slats align flush with the support tops. Secure with stainless steel screws—these will hold your cushion. Space 2" x 2" seat slats every six inches and screw them into the slat support brackets with 2" stainless steel screws.
Position the 2" x 4" x 3' armrests on the 4" x 4" posts, about 18" above the seat supports (adjust for cushion depth). Fasten to the inside of the posts using 3" screws.
Here are two proven methods to attach the top rails to the 4" x 4" posts:
Option 1: Cut notches in the rails and matching mortises in the posts. Insert, apply glue, screw, and clamp.
Option 2: Screw the crosspieces directly to the inside or outside of the 4x4 posts.
Fit caps onto the 4" x 4" post tops and clamp securely in place.
Brush on a thin coat of semi-transparent penetrating stain for lasting protection. Let it dry completely.
