We constructed a king-size bed, adjust the lumber sizes to match the size of your bed.
First thing to build for the bed is the cross section. Start by building four I beams with 3/4" maple plywood at 6-3/4" x 80-1/2". Take your caps and place them over the center of each end of the vertical piece. When you look at these from the side, they form an "I" shape. Set two of these I-beams aside.
Make two U beams by using 3/4" maple plywood at 6-3/4" x 80-1/2". Add caps to the vertical pieces, only this time, make sure the caps are flush to one side. When you look at these from the side, they form an "U" shape. Set one of the U beams to the side.
Start putting the frame together by building the bed frame in two separate pieces, so it will be easier to move to the bedroom. Lay the I and U beams out with the U beam on the end, flush side facing outwards. Place an I beam 11-1/4 inches from the U beam, and then take the second I beam and place it 12-1/2 inches on center from that.
Take one of the facing beams and place it across the end of the I and U beams, securing it with glue and nails. Repeat this process at the opposite end. Take two cap pieces and attach them between the two facing beams, one on top and one on bottom.
Take the center vertical piece and attach it to the open end, securing it to the two cap pieces you just added. Repeat the steps above to make the second matching frame. Paint the outside of the frame your desired color.
To make the trim for the bed, make a three-inch rabbet cut at 1-1/2 inches deep. For the side pieces, start two inches from one end and cut 88 inches down one side of the trim piece. Stop short six inches from the opposite end. For the foot trim piece, make the rabbet cut all the way down one side. Paint the trim your desired color.
Cut out the platform pieces and attach the side platforms to both frame pieces using countersink screws. Paint the edges of the platforms.
To assemble the bed frame, add the trim by placing the rabbet cut over the edge of the platform, making the trim flush with the platform. The trim should stick out 6 inches from the foot of the bed frame. Drill pilot holes from underneath the trim and attach with countersink screws.
Once the trim is attached, take the two frame pieces and attach them together in the center using countersink screws.
To finish, put the center platform in place and attach it using countersink screws.