Measure the frame of your workbench with a tape measure. Record these measurements and keep them on hand.
Mark the dimensions on your old door or plywood sheet using a black marker. These markings will guide you as you cut the top to size.
Place the workbench top on a hard, flat work surface, such as a concrete floor or work table. Place your circular saw with saw guide on the first cut markings. Saw your workbench top to fit the frame on each side.
Attach your workbench top to the frame. Drill four holes through the top of the workbench and into the frame at each corner of the table. Screw your workbench top into the frame and test for sturdiness. Attach with extra screws along the edges of the workbench top, if desired.
Sand the surface of the workbench top with 200-grit sandpaper. Apply a sealant or a laminate over the sanded surface and let dry. Apple one or two more coats, depending on how slick and even you want the surface to be. Let the laminate dry overnight.