Determine the height of the workbench by deciding if you want to stand or sit at it. The common height of a sit-down workbench is 30 inches.
Using a square, line up one of the legs to a cross beam. At the top of the leg and cross beam, pre-drill holes for screws. Attach screws.
On the side of the leg, pre-drill two holes for two 4-inch lag bolts. Insert the lag bolts and round washers. Set the lag bolts and washers farther into the wood. Repeat for all four legs.
Attach support pieces to middle of legs. Repeat step 4 with square washers and 8” lag screws to connect the front and back of workbench. The lag bolts and washers will sit flush to the wood. Reinforce intersection with screws.