Draw a line from corner to corner on one of the 4-inch boards. Drill a 2-inch hole where the lines intersect. The barrel of your gun will fit through this hole.
Position the 4-inch boards on edge, parallel and 34 inches apart. Lay the 36-inch board on top of them so the edges are flush. Drill 3/32-inch holes through 36-inch board and into the 4-inch boards. There should be two holes that enter each 4-inch board at the 1/2-inch center on the edge. Space the holes 2 inches apart. These are pilot holes for the 2-inch screws.
Drill a 1/4-inch-deep, 3/8-inch hole into each pilot hole on the 36-inch board. This will hide the screw heads when the gun holder is mounted on the wall. Screw the boards together using the 2-inch screws.
Drill two more pilot holes through the 36-inch board. There should be a hole at each end, 2 inches away from the ends and 2 inches away from each edge. Drill a 1/4-inch-deep, 3/8-inch hole into each of these holes.
Locate a stud in the wall where you would like to hang your gun holder. Position your gun holder against the wall vertically, with the 4-inch board that has the hole on top and so both 4-inch boards are parallel with the floor. Use your level to ensure these are true. Screw the holder to the wall using the two 4-inch screws and the pilot holes drill in the previous step.