Sand the treated board 2-inches-by-10-inches-by-6-feet that you will use for the picnic bench seat if necessary. Pay special attention to the sides and top that can splinter off into unsuspecting legs.
Cut the treated Board 2-inches-by-4-inches-by-8-feet into 2-foot sections that you will use for the legs of the bench.
Lay one 2-foot board on top of another, crossing them in the center to create an X. Be sure the top outer edges of the X does not exceed the 10-inch width of your seat, since these are the legs of your picnic bench. Repeat this step for the second set of legs.
Place a bracket over the crosspoint of the boards and drill a hole in the center for the bolt. Place the bolt through the hole, place a bracket on the other side and secure with a nut. Repeat this step for the second set of legs. Drill a hole at the top of each leg for the bolt to attach the legs to the seat.
Drill four holes through the bench seat to attach the legs using bolts. Set the legs two-to-four-inches in from the ends of the bench. Keep in mind that one bolt will be two inches further in than the other. The bolts should also be positioned at least one-inch from the outer edges to prevent cracking the seat. Be sure that the bolt heads are flat enough not to make the seat uncomfortable.