Put one sawhorse bracket on each end of each 4-foot 2-by-4, following the directions of the manufacturer of the sawhorse bracket. Secure the brackets to the 2-by-4s with wood screws. This is the top of the sawhorse.
Insert the remaining 2-by-4 boards into the leg slots of the sawhorse brackets, following the manufacturer's instructions. These sawhorses will be both the legs and the support brackets for the top of the workbench.
Line up two sawhorses 8 feet apart, with their ends facing you. Place the remaining three sawhorses evenly spaced between the two end ones.
Lay the piece of plywood on top of the lined up sawhorses. Align the edges of the plywood with the top edges of the end sawhorses. Align the middle sawhorses so the edges of their top boards are even with the edges of the plywood.
Attach the top boards of the sawhorses to the plywood from the bottom of the sawhorse top board, through it and into the underside of the plywood. This way the screws will be invisible and the top of the workbench will be smooth and free of screw heads. Use at least three screws for each sawhorse: one on each end and one placed midway between them.