Build a twin platform bed using stock lumber by building a box from 2x4s, adding the legs, cleats and slats and finishing the bed. Predrill all screw holes to prevent splitting the wood. Larger beds require providing center support for the slats.
Build the support box by cutting two 2x4s 76 1/2 inches long and two 39 inch 2x4s. Attach the 39-inch boards inside the longer ones by countersinking 3-inch wood screws. Measure the diagonals and adjust the box so they are equal to ensure it is square.
Add 5 1/2-inch legs set 1/2 inch below the top of the frame to prevent the mattress from sliding off the platform. Attach 2x2 cleats cut to fit between the legs 1/2 inch below the top of the frame using wood glue and screws.
Using glue and screws, attach 1x3 lumber to the top of the frame horizontally so it is flush with the inside of the frame. Do not attach a lip across the head of the bed if using a headboard. Attach 1x3 slats to the frame, screwing them to the cleats. The exact number of cleats and spacing depends on the quality of the mattress.
Finish the bed by sanding so it is smooth. Apply primer and paint or wood sealer, stain and polyurethane for the preferred finish.