Set the 2 x 4 x 60-inch boards and 2 of the 2 x 4 x 12-inch boards into the form of a rectangle, with the ends of the longer boards butting against the sides of the smaller boards. Drill 2 screws through the sides of the short boards into the ends of the long boards. Place the 2 x 4 x 8-inch boards between the long boards,15 inches in from the ends. Drill screws through the long boards into the ends of the 8-inch boards. This creates the frame of the garden slat bench.
Place 2 of the 2 x 6 x 24-inch boards flat against a short side of the frame. Even up the edges. Drill 2 screws through each 2 x 6-inch board into the sides of the frame. Repeat this process for the opposite side. This creates the legs of the garden slat bench.
Use the measuring tape to measure 3 inches up the inside face of the legs from the bottom of the frame. This should leave you 7 inches remaining of the leg. Do this with both legs and make a line at that point. Measure up the face another 2 inches. Make a line at that point on both legs.
Attach a 2 x 4 x 12-inch board across the inside of each leg between the lines. Drill 3 screws through the outside face of the legs into the cross piece so no sharp points are exposed.
Lay the 2 x 4 x 56-inch board between the legs. Rest the board on the 4-inch side on the 2 cross pieces so that it sits in the middle between the boards forming the legs. Drill 2 screws through this cross member into the smaller boards, with the screws being close to the sides.
Place the remaining 2 2 x 4 x 12-inch boards across the inside face of the legs as you did the previous 12-inch boards. Set these 2 boards to rest upon the 56-inch cross beam. Line up the ends with the boards below them. Drill screws through the outside face of the legs into these boards. This creates the support structure of the garden slat bench.
Flip the entire bench over so it stands on the legs. Line up the six 1 x 2 x 60-inch slats on top of the frame. Make the ends of the boards flush with each other and the frame. Drill screws through the slats into the frame. Use 1 screw per slat where it contacts the frame; use 4 screws per slat.
Sand down the corners of the seat with the sandpaper.