Measure the distance between the outside edges of two parallel ceiling joists at the location where you want to install the ceiling shelf.
Cut a piece of 1/2-inch plywood to match the width you measured in the previous step. Cut the board to 4 feet in length.
Cut two 2-by-4-inch boards to match the width of the plywood board. Place the boards under the plywood board, 1 foot from the ends of the board, extending across the width of the board. Nail the boards together with 10d nails, spaced every 6 inches.
Cut four 14-inch 2-by-4-inch boards. Place these perpendicular to the 2-by-4-inch boards nailed to the plywood and nail them to the ends of the boards with 10d nails.
Have a helper or two lift the shelf up and position it so the vertical 2-by-4-inch boards overlap the joists by 2 inches. Nail the boards to the joists, and the shelf will hang from the ceiling.