Measure the length and height of the portion of the kitchen wall along which your bench will sit. Decide how deep your bench should be. This depth is the distance that it extends out from the wall.
Cut the lumber on a table saw so that you have two pieces the exact length of your bench seat and 1 inch short of the finished height, and two pieces the exact depth of your bench seat and 1 inch shorter than the finished height. Build a rectangular box from these four pieces, keeping the edges of the shorter pieces to the inside to form precise 90-degree corners. Screw the boards together using drywall screws spaced 2 to 4 inches apart.
Cut a piece of plywood to the exact top dimensions of your box. Lay the plywood in place over one open side and screw it to all four edges, spacing the screws 2 to 4 inches apart. Fill all of the screw holes with wood putty and let it dry for several hours.
Sand the entire bench to a smooth finish. Stain or paint the bench and add any molding or other decoration that you want. Let the bench dry completely.
Remove nails from any baseboard molding along the wall for the bench. Pry the molding away from the wall with a crowbar. Set the bench in place with the back flat against the wall and the open side face-down.
Attach the bench banquette to the wall using L-brackets and screws. Add decorative pillows or a banquette cushion to make the bench more comfortable for sitting. Arrange your table in front of the bench seat.