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Decide where you want the two end brackets to go, and remove the old railing.
Measure the space between the brackets to determine even spacing in-between and fasten the brackets with screws.
Attach 2 x 4's to each bracket for back support. First, cut an angle at the top of each board to create a level edge. When measuring for an angled cut, use sliding "t" level butted against support as shown in the image; use that angle to adjust your circular saw at an angle. Cut the top of the 2 x 4's and clamp them to each support; level, drill holes to match the support bracket holes, and attach with carriage bolts.
Attach the boards for the seat of the bench. First, lay the back 2 x 4 board across the back of the brackets, flush to the outside of the deck. Lay two more boards flat across the brackets. Attach another board across the front of the four brackets to create an overhanging trim piece or "skirt."
For the top of bench, set the table saw for the same angle cut used on the support boards to create a level board at shoulder height for setting beverages on. Clamp the cut board in place using a framing square to make sure it's flush, and then screw it on. Attach the rest of the back bench boards, making sure they are level.