Cut the sheet of plywood diagonally. Start at 24 inches up on the 8-foot side. The diagonal cut ends 24 inches from the end of the other side of the wood. Cut the 18-inch trim into four equal lengths.
Measure the height of the filing cabinets. Cut the 1-by-8 board into two equal pieces. For instance, if your filing cabins are 2 feet height, the 1-by-8 board needs to be 4 feet long. After it is cut, it will be in two 2-feet pieces.
Paint the plywood and 1-by-8 boards and allow them to dry.
Create a "V" shape with the 1-by-8 boards. Screw them together. Place them in the corner with the point facing out.
Place the plywood upside down on the floor. Fit the diagonal cuts together to create the "L." Attach the loop lock and the "V" bracket to the area where the boards connect. This braces them together. Place the boards on the corner 1-by-8 piece. Slide the filing cabinets under each end. The easiest way to do this is to have someone hold the plywood while someone else slides in the cabinets.
Mark the underside of the plywood with the width of each cabinet. Mount one trim piece to each side of the filing cabinet with two screws.
Attach the wood edging around the outside of the desk. Iron around the edge to make the adhesive stick to the wood.