Ensure that all of the lumber ends are square to ensure the measurements will be accurate.
Cut the plywood so that it is 38 inches wide and 75 inches long to fit a twin-sized mattress for the child's four poster bed. Use what remains to make two 8 x 39-inch side pieces.
Make a notch in the corners of the bed board so that you can fit in the bed posts, then cut 2 ½-inch notches from the inside of each posts to accommodate the mattress.
Lay two of the posts horizontally on the ground so that they're 75 inches apart.
Decide how high you would like to have the mattress from the floor and position the sideboard on the posts at that height. Make sure the side boards and the posts are even and square, then drill pilot holes, and secure the sideboard in place with 1 ½-inch #10 screws. Repeat the procedure with the other posts.
Glue and nail the rail that goes between the bed posts. The rail should be 2 ½ to 3 inches lower than the top of the side rail. Its length depends on the mattress width. Repeat this step for the end of the bed, except substitute 1 ½-inch #10 screws for the nails.
Set the plywood sheet that supports the twin mattress on the child's four poster bed. Glue the wide finials to the top of the bed posts. You can add decorative head- and foot-boards to the bed if you'd like.
Sand the bed and round any sharp edges. Dust it thoroughly, then apply polyurethane or paint. For a finishing touch, install curtain rods in between the bed posts and hang the curtain of your child's choice.