You will need the following things to get started.
Box (I used a Postal one, but a Cereal Box or Cracker box could also work)
Utility knife or Craft Knife
Tape, Masking or Gaff
Material (for mattress cover)
Scrap Soft Cell Foam (Mine is leftover from a mattress topper)
Step 1: Prepare Box
Step 2: Design and Cut out Headboard
Cut out the cardboard headboard and footboard.
Step 3: Make mattress
If you don't have foam you can stuff the mattress cover with something like Poly-fill also.
Step 4: Attach Headboard and Footboard
I also taped all around my corners on the Headboard to cover the holes. Then I taped across the whole thing, to add some strength.
Step 5: Paint the bed
The finished paint job is shown. I used two colors.