Sketch the backboard to organize the construction process and to calculate its size. Draw a rectangular board with two legs that will be screwed to the daybed frame. Measure and note the length of the daybed frame. Note the desired overall height of the backboard and the height of the assembled daybed. The lower edge of the backboard is typically just above the upper surface of the mattress.
Add 1/4 inch to the length of the bed to calculate the length of the backboard. This will allow the backboard to fit easily onto the frame. Measure and mark a 1-by-4-inch plank to that dimension, and use a circular saw to make a 90-degree cut. Repeat to trim a second plank with the same dimensions. Measure, mark and cut two more planks, each to the desired height of the backboard.
Assemble the frame on a flat surface. Lay the vertical framing members parallel to each other. Lay the top horizontal framing plank on top of the vertical planks. Align the top and side edges of the horizontal plank with the upper ends of the vertical planks. Use a carpenter's square to square both corners. Use a drill and driving bit to screw the horizontal planks to the vertical planks with drywall screws. Each joint should be affixed with four evenly spaced screws.
Refer to dimensions on your sketch to mark the vertical planks where the lower edge of the backboard will be positioned. Lay the lower horizontal framing member across the vertical framing members, aligning its lower edge to the positioning marks. Square the joints, then screw the horizontal plank to the vertical framing members.
Refer to your sketch to mark the plywood sheet to the size of the backboard. To conceal the frame, you may want to cut the plywood so it is slightly larger than the frame. Use the circular saw to cut the plywood to size. If you plan to upholster the backboard, do so before you attach the frame.
Lay the plywood on a flat surface. Position the frame on the plywood with its horizontal framing members against the plywood. Align the edges. Use 1-inch drywall screws to attach the horizontal framing members to the back of the plywood. The screws should be evenly spaced, about 6 inches apart.
Measure, mark and cut two 1-by-4-inch planks, each 12 inches long. Mark a curved cutting line on one corner of each plank, using a drawing compass. Trim the planks on the curved cutting lines, using a jigsaw. The 12-inch planks will be attached to the bottom of the frame, where the legs rest on the floor. They provide an attachment for the daybed rails.
Lay the backboard on a flat surface, front-side up. Position the 12-inch planks on the outer edges of the lower ends of the legs of the backboard frame. Align the straight long edges of the 12-inch planks to the back edges of the legs of the frame. The straight bottom edges of the planks are perpendicular and aligned to
the lower ends of the vertical framing members. Use 2-inch drywall screws to attach the planks to the frame's legs. The screws should be evenly spaced, about 3 inches apart.