Start with 2 of the pieces C, the 1×4 trim pieces. Begin by lining up the panel pieces for the headboard, A, on top of the 2 trim pieces, C, as shown above. Use glue and 1 1/4″ nails to nail through the panel pieces, A, into the back side of C. Check for square (visit the HOW-TO section to learn how to check for square). Nail all 11 headboard pieces to the back side of C.

Do the same for the footboard, this time using the panel pieces B, and two more of the 1×4 trim pieces, C.

Your Headboard and Footboard panels should look like the above diagram. Make sure these panels are square.

Begin with piece D, and cut out with a jigsaw the desired arch. The arch used above begins 5 1/2″ on either end and reaches up 6″ at the highest point in the center. After the arch is cut out of the header piece, D, for the headboard, flip the larger panel over, as shown above, and secure the header piece, D, as shown above, using 2″ nails and glue. Keep all outside edges flush.

Mark the 4×4 posts 5/8″ in on either end on the side that will be connected to the center panel. Line up the center panel with these marks to center the panel on the 4×4 post as shown above. Predrill 1/2 way through the post and then screw with 4 1/2″ screws through the predrilled hole into the center panel. Make sure you are lining up your screws with the panel where it is sandwiched 3 boards thick.
Do the same with the footboard pieces, using the shorter posts and the footboard panel.

Then add H, the 2×6 top piece, as shown above, over pieces G, to both the headboard and footboard. There will be a 1″ overhand on all sides.

Attach the cleats (I), as shown above, with the bottom edge 9 1/2″ from the bottom. Predrill and use long screws to secure, 4 screws per cleat.

Next add the end piece for the cleats, J, inset to the cleats as shown above. Use long screws, 3 per end, predrilling through the cleat (I) into the end edges of J.

Now position the footboard so that piece J lays flush with the panel piece, as shown above. Use 2″ screws to screw through J into the footboard, screwing all the way down J. Make sure the cleats are level.

Here is a view from behind the headboard.

Add the siderail pieces, keeping the bottom edge of the siderail, K, flush with the cleats (I). Use screws to screw from the inside of the cleat, through the cleat and into the siderail. Predrill and use 2″ screws, screwing down the length of the siderail.