Cut the tongue off of one of the tongue-and-groove boards. Sand the edge smooth. Set this aside for the tabletop.
Cut the 8-foot 2-by-4 into two 30-inch and two 18-inch pieces. Set the four pieces on edge with the shorter pieces inside the ends of the longer ones to form a 21-by-30-inch tabletop base. Using a 1/16-inch bit, pre-drill holes for two screws at each end of the longer boards into the ends of the 18-inch pieces. Glue and screw the pieces together.
Cut the 6-foot 1-by-6 board exactly in half.
Drill the holes and then glue and screw these 36-inch pieces flush with the end of the base and centered so that they extend out 7 1/2 inches beyond the short sides of the base. The top pieces will be glued and screwed to these two pieces.
Place adhesive on top of the two 36-inch boards. Place the six 48-inch tongue-and-groove pieces across the top of those boards. Each end should extend evenly about nine inches beyond the base assembly. Five pieces will fit together with the tongue in the groove of the next piece; the final piece is the one with the tongue cut off. Adjust the pieces so the ends of the tabletop are even and so that the 36-inch pieces do not stick out beyond the tabletop. Mark an erasable line across all six pieces that runs along the center of the 36-inch pieces. Measure and mark two points to attach each tongue and groove piece. These should be evenly spaced 1 inch in from each edge of the tongue and groove material so as not to split the lumber when driving screws in the next step.
Drill 1/16-inch holes at these 24 points. Drive drywall screws just below the surface without cracking the plastic.
Cut four legs from the 7-foot 2-by-4 boards to 40.5 inches long each; you will have two legs per board.
Flip the table on its top. Place the ends of the legs in each corner of the base with the 4-inch sides against the longer side of the base. Glue them to the base. Drill four 1/16-inch holes through the side of the base into each leg. Attach the legs with drywall screws.
Cut two 30-inch pieces from the 5-foot 1-by-6 board. Cut two 21-inch pieces from the 4-foot 1-by-6.
Glue and screw these around the legs -- two screws at each end -- so that the top edges of these pieces are 10 inches above the floor. They brace the table legs at the bottom and serve as a footrest.