1. Purchase the mattress. A standard twin mattress measure approximately 39 inches by 75 inches, but the size can vary slightly from one manufacturer to another. You may need to adjust the size of the swing to fit your mattress.
2. Choose good-quality wood. For ease of construction, stock wood sizes such as 2x2, 2x4, and 2x6 are used. (Note that nominal wood sizes are bigger than the actual size of the wood. For example, a nominal 2x4 measures only 1-1/2 x 3-1/2 inches.) Select straight, warp-free pieces that are free of knots or other defects.
3. Make sides first. Begin by cutting four 2x4 legs, each 26 inches long. Cut four 2x6 apron pieces, each 31 inches long. (These are the horizontal boards -- top and bottom -- on the sides.) Attach two of the legs to two of the aprons using wood glue and lag screws. Pre-drill for the screws to prevent splitting the wood. Use carpenter's square to check all angles. Countersink the screw heads and plug. Note that the top apron is attached two inches down from the top of the legs and the lower apron two inches above the bottom of the legs. Make the second end to match the first. Allow glue to dry.
You have a good start on your project; now you're ready to finish it off.
4. Put it all together. Cut three 2x6 rails, each 75 inches long. With glue and lag screws, attach one rail to the front of the two sides in line with the lower side aprons. Attach the two back rails to line up with the upper and lower aprons. Stagger the screw holes to miss the other lag screws. Check that everything is square and allow glue to dry.
5. Cut 2x2 pickets to fit between the upper and lower aprons approximately 15 inches long. Space the pickets evenly, using eight on each end and the remainder across the back. Install with glue and finishing nails. Countersink and fill all nail holes.
6. Add supports for the seat. Cut two 2x2 ledgers 75 inches long. Glue and nail these on the inside of the front and rear lower aprons, exactly 3/4 inch from the top of the aprons. Cut six 1x6 slats 31 inches long and install on top of the ledgers.
7. Sand the entire piece smooth. Paint or stain the piece as desired. Light stains work best on pine. For an easy, no-drip job, use one of the new gel stains. Finish with two or three coats of clear polyurethane. For a painted finish, prime first, then apply two coats of exterior latex enamel.
8. To hang the swing, install four eyebolts in the upper side aprons, a few inches in from the legs. (Make sure the bolts miss the pickets.) Position the swing on the porch. Attach four more eyebolts to ceiling joists directly over swing eyebolts. If there are no joists correctly positioned, add 2x6 braces spanning existing joists and run eyebolts through the ceiling into braces. Never anchor directly to ceiling paneling or drywall. Make up four equal lengths of heavy chain to hold the swing about 3 inches off the floor.