Lay the vertical planks for the gate face down in their final configuration on a hard, level surface. Ensure the boards are already cut to size and shaped.
Cut two identical lengths of 2-by-4-inch lumber that match the width of the tops of the vertical planks for the gate. Sand the ends of the two boards so they are smooth. The two planks form the horizontal braces for the gate.
Lay one board horizontally over the vertical planks, 4 to 6 inches from the ends. Ensure the ends of this board fall flush with the ends of the corner planks or trim excess length if required. Drill a row of pilot holes every 3 to 5 inches along the horizontal board and into the corresponding vertical planks.
Lay the second board horizontally over the vertical planks, 4 to 6 inches above their ends. Repeat the process of drilling pilot holes along the span of the board and into the corresponding vertical plank.
Drive a 3-inch galvanized builder’s screw into each pilot hole to join the horizontal brace to the vertical gate planks. Cut a length of 2-by-4-inch lumber to match the space between both the horizontal braces.
Lay the 2-by-4-inch board diagonally over the horizontal braces so it extends from the upper right corner of the assembly to the lower left corner. Form a dotted line along the upper and lower end of this diagonal brace where it touches the corresponding horizontal braces, thereby forming cutting guides.
Cut along the dotted line at each end with a saw. Insert the diagonal brace between the horizontal braces, or trim excess length along the corners, if required, so it forms a tight fit. Predrill three holes through the upper and lower horizontal braces and into the ends of the diagonal brace.
Drive 3-inch builder’s screws into the pilot holes to secure the brace to the gate, thereby reinforcing or strengthening it.