Saw a hardwood two-by-four board to a length of 36 inches. Drill a 1/2-inch hole through the 4-inch face of the board, 2 inches from one end. Drill another hole 2 inches from the other end. Now drill a 1/4-inch hole in the center of the board.
Place a 3/8-inch steel clip-type carabiner in each hole on the ends of the board. Place a 3/8-inch eye bolt at least 3 inches long in the center hole, and attach the nut on one side. Tighten the nut with a wrench.
Locate a ceiling joist where you want to hang the string chair. Drill a 1/4-inch pilot hole into the joist. Screw a 3/8-inch eye bolt into the ceiling joist.
Hook one end of a heavy-duty spring in the eye of the bolt anchored in the ceiling joist. Hook the eye of the bolt in the middle of the hardwood board on the other end of the spring.
Slip one end ring of a small string hammock into one of the carabiners in one end of the board. Slide the other end ring into the other carabiner. Do not use the type of hammock that has wooden spreaders on the ends.