Cut the 2-by-4 lumber into two lengths of 54 inches and two lengths of 34 inches, then use the miter box to trim the ends to 45-degree angles. Cut six planks more to 30 inches. Cut the plywood into a 34-by-54-inch piece and a 30-by-36-inch piece. Sand all the components.
Form the planks with the mitered ends into a rectangular frame. The 45-degree angled ends will mate to form 90-degree corners. Use one wood screw through each side of the corners to screw them together -- 8 screws in total for securing the frame together.
Place the 34-by-54-inch plywood sheet over the top and secure it into the frame with three screws evenly spaced along the shorter sides and four evenly spaced along the longer sides.
Turn the frame upside down so the plywood is flat on the work surface. Clamp four of the 30-inch planks into each corner of the frame -- the 4-inch face of the planks should be flush with the 34-inch side of the frames and the planks should jut upward at right angles. These are the legs of the garden worktable.
Drive two screws through the shorter sides of the frame into the legs and two through the longer side into the legs -- use four screws all together for each leg. Turn the table back upright, and rest it on the legs.
Position the remaining two 30-inch planks across the legs 5 inches from the ground to create the cleats, or supports of the table's under-shelf. Secure the cleats in place with two screws through the cleats into each leg.
Place the 30-by-36-inch plywood piece over the cleats and secure in place with three screws evenly spaced through the top of the shelf into the cleats.
Varnish or paint the garden worktable, applying two coats. Follow the manufacturer's recommended drying times between coats.