Remove the temporary screw you drilled to make the U-shape at one end of the garden bed. Lay two sleepers down one side. Drill a temporary screw to join the sleeper at the other end, so it makes a corner. Use the roofing square to make sure that the corner is square. Once it is square, lay the two other sleepers on the opposite side. Check that all four corners, where the sleepers join, are square. Use marking paint to mark the corners on the outside of the sleepers, which will tell you where to dig a trench for the sleepers.