After gathering all the tools you will need for the job--circular or table saw, drill, screws, hammer, level, straightedge, pencil, sander, router, and safety goggles and gloves--lay out your plywood to begin outlining the layout of your work surface. Use a thick, floorboard-quality plywood that is durable enough to last without bending--3/4 to 1 inch in thickness. Depending on how heavy you want your work surface to feel, you can double up the layers of plywood and wood glue them together for a statelier look.
A 4-foot by 8-foot floorboard plywood sheet may be large enough to cut a one-piece, L-shaped desk about 4 feet one way and 8 feet the other way. If you prefer an even larger desk, then the work surface will be constructed of more than one piece of wood. Determine how deep you want your work surface to be. You may find that 2 to 2 1/2 feet wide is sufficient. Transfer your measurements and any additional decorative designs onto the plywood with a thick pencil line.
For straight cuts, use a circular or table saw. A table saw is better for an even and smooth cut, but a steady set of hands can yield similar results with a circular saw. Curved corners are accomplished using a jigsaw. Carefully follow the lines and then sand away any rough edges all around the work surface top. Leave the edges straight or router a decorative edge into the wood. If your work surface does not have curved edges, trim the edges with a decorative molding instead of routering a design into the wood. Sand the surface and edges smooth.