Select a pair of identical twin headboards. You may be able to use a pair of headboards you have around the house or find them at garage sales and thrift stores for surprisingly low prices. Before you buy, check the headboards to see whether they are in decent condition.
Measure the width of the leg on one of your headboards, double it and add it to the length of your twin mattress. Add an additional 3 to 4 inches so the mattress can fit comfortably within the frame.
Cut two pieces of 1-by-6 pine to the measurement just taken. Cut two pieces of 1-by-2 pine, using the same measurement minus the doubled width of the headboard legs.
Lay the 1-by-6 boards flat on the ground and lay one of the 1-by-2 boards on top of each one, aligning the bottom edges. The 1-by-2 boards should be several inches shorter than the 1-by-6s. Position them so they are centered on the longer boards.
Pre-drill holes through the 1-by-2 boards, then screw them to the 1-by-6 boards, using standard wood screws. Place one screw at each end and every 6 to 8 inches in between. These sets of boards will act as the sideboards for your daybed frame.
Pre-drill two holes approximately 4 inches apart near each end of the the two sideboards. Determine how high you want your mattress to sit off the ground and pre-drill two holes, spaced 4 inches apart, at the appropriate height on each headboard leg. Install the sideboards by inserting 2 1/4-inch round-head wood screws into each pre-drilled hole.
Measure the width of your completed daybed frame and cut a number of 1/2-inch wood slats to that measurement. Install the wood slats across the width of the frame, nailing them to the 1-by-2-inch supports, at 8-inch intervals, to serve as mattress supports.
Set your twin mattress inside the daybed frame and make the bed as you desire. If you plan to use the daybed for sleeping as well as seating, you may want to cover the mattress with a mattress pad and fitted sheet before covering it with a flat sheet and duvet or comforter. Arrange a number of matching throw pillows along the back of the daybed where it rests against the wall.