Set one 12-inch strip and one 22-inch strip together in an L-shape. Glue them together with the end of the 22-inch strip abutting the inside face of the 12-inch strip. Glue them in place, and then reinforce them with two nails from the nail gun.
Set the remaining 12-inch strip on the free end of the 22-inch strip. Glue and nail as you did the corner of the "L," creating a squared "C."
Slide the remaining 22-inch strip inside the open face of the "C." Position it parallel to the other 22-inch strip, 2 inches from the open ends of the 12-inch strips. Glue and nail each end as you did the other connections.
Set one wooden plank on top of your frame, its edges aligned with those of the frame. Glue it in position, and then reinforce it with a line of six evenly spaced nails along each edge. Repeat to set the other plank on the opposite side of the frame.
Wipe off any excess glue and allow it to dry.
Paint all faces of the frame and allow it to dry. While it's drying, set it on the open face.
Hanging the Brace
Find two wall studs on the wall where you want to hang your frame. Mark their location with a pencil.
Set your level so it crosses both studs. Level it out, and then trace a line along one edge of the level.
Set your 2-inch by 2-inch beam with one edge lying along the line you drew. Mount it to the wall by driving one 4-inch wood screw through the beam and into each of the wall studs.
Hanging the Shelf
Slide the open end of your shelf frame over the beam you mounted to the wall.
Attach the shelf to the beam by driving the three shorter wood screws through the plank of the shelf and into the side of the beam. If the shelf is above eye level, do this through the top of the shelf. If it is below eye level, do it through the bottom.
Add touch-up paint to the visible screw heads and to any dings that occurred during construction.