Arrange the 2-by-8-inch boards in a rectangle. Place the ends of the short boards aligned with the sides of the long boards.
Predrill two 1/2-inch holes through each side of the long boards into the ends of the short boards. Insert the tip of the 3/4-inch spade bit into the holes and drill 1/2 inch into each hole. Use a socket set to attach the boards together with lag screws. This is the base of the daybed.
Predrill and screw a corner caster into each corner of the daybed. Turn the daybed over.
Align the edges of the plywood on top of the daybed. Predrill every 6 inches through the plywood into the top edges of the wood frame. Screw together with deck screws.
Sand the entire daybed with 180-grit sandpaper to remove any splinters and rough edges. Wipe with a tack cloth to remove dust.
Stain the daybed with a tint that matches or coordinates with your king-size headboard. Rub the stain over the wood using a rag. Remove the excess stain and allow to dry completely. Brush two coats of polyurethane over the daybed, allowing it to dry between coats according to manufacturer directions.
Align the headboard on the back of the daybed. Predrill two holes through each headboard support and completely through the wood frame. Screw the headboard to the daybed with the four lag bolts that are 1/2-by-5 inches. Tighten the lag bolt nuts using a socket set.
Place a twin mattress on the frame Make up the bed and add cushions to the back of the daybed.