Hold up two 2-by-2-by-36-inch leg boards with the help of an assistant. Bridge the two legs by placing a 1/2-by-4-by-24-inch support board across them. Drill two holes from the top of the support board, plunging down into the tops of the legs. Slide 2-inch wood screws into the exposed holes. Tighten the wood screws into the holes using the drill. Do the same for two other legs and a support board.
Turn the countertop upside down and position the assembled leg portions evenly on the underside of the countertop. Drill four holes in each leg portion, through the top of the support board and down through the countertop, taking great care not to emerge on the other side of the countertop.
Put 2-inch wood screws in each hole and tighten them completely, being careful not to puncture the top surface of the countertop.
Stand two 2-by-2-by-36-inch leg boards using an assistant. Bridge the two legs by placing a 1/2-by-4-by-24-inch leg attachment board across them. Make two holes from the top of the support board and going straight down into the tops of the legs. Set 2-inch wood screws into the open holes. Completely fasten the boards together by tightening the screws, then repeat for two additional legs and a leg attachment board.
Turn the board that is slightly smaller than the countertop upside down and evenly position the two leg attachment boards on top of it with the legs pointing upward. Make four holes through each leg attachment board and into the other board. Attach the boards together with 2-inch wood screws.
Right the assembled pieces with the legs pointing downward and smear an adhesive recommended for wood and the countertop material all over the top surface. Position the countertop on the board evenly an allow time to set.