Place the two 18-by-40-by-3/4-inch plywood on a worktable. Measure from one end and make a mark at 12 inches. Repeat this from the other end as well. Place the framing square on the marks and draw lines across the width of the plywood. Next make an "X" to the left of one mark and to the right of the other.
Apply glue to the ends of the four pieces measuring 18 by 401/5 by 3/4 of an inch, and secure two of them on the ends of the pieces from Step 1 with the drywall screws. Secure the other two pieces on the lines with the "X's." This is the wall support for the bar top. Clean up excess glue immediately with a damp cloth.
Place the two pieces of plywood measuring 36 by 48 by 3/4 of an inch on the worktable. Turn the best-looking side down. Apply glue to the top of one and secure the second to it with the drywall screws. Don't allow the screws to overpenetrate.
Apply glue to the top of the wall. Place the assembly from Step 3 on top of the wall allowing it to hang over 4 inches on the ends and 16 inches on one of the long edges. Secure it to the wall with the drywall screws.
Install the two pieces measuring 42 by 40 by 1/4 of an inch to the front and back of the bar wall with the paneling nails.