Measure six spots spaced an inch apart on each PVC pipe and drill a 1/4-inch hole through each pipe with these measurements. The two outside holes will be an inch away from the ends of the pipe.
Glue the 4-inch pipe into the 4-inch bracket and glue the 3 1/2-inch pipe into the 3 1/2-inch bracket. Let the glue dry according to the instructions on the container.
Position each bracket on top of the plywood so the holes are at a right angle to any pair of parallel edges. Center the brackets using your tape measure. The distance from the base of the bracket is the same distance from the edges of the plywood around each bracket. Screw the bracket to the plywood in this position. The plywood with the 4-inch bracket is the base to the adjustable monitor stand.
Insert the 3 1/2-inch pipe into the 4-inch pipe. Align the holes in the pipes and adjust your stand to your desire height. Insert your metal pin into the stand to secure the stand at this height.