Sand one face of the plywood sheet until it is furniture smooth. This may require several passes with increasingly fine grains of sandpaper.
Set the sawhorses on a level surface, parallel and 36 inches apart, with their ends aligned.
Place the plywood sheet on the sawhorses, sanded side up. The short ends of the plywood should run parallel to the sawhorses, and each overlap the sawhorses by 12 inches. One long end should be flush with the ends of the sawhorses, with the other overlapping by approximately 6 inches.
Screw the plywood to the sawhorses using two screws per 'horse. Using screws instead of nails makes disassembling the desk significantly easier.
Move your desk into place in the room where you intend to use it.