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
Prepare your box by removing any shiny tape. Then cover with a layer of tape to reinforce it.
Step 2: Design and Cut out Headboard
Make a headboard design. Draw it onto some paper and cut out the template. Then trace the template onto your cardboard. Make sure to leave enough room at the bottom to allow for the mattress.
Cut out the cardboard headboard and footboard.
Step 3: Make mattress
Cut your Soft Cell Foam to the size of your box. I had to split mine with scissors because it was too thick. This is easy to do by just cutting it in half with scissors all the way across. I had to glue my two pieces together with fabric glue. I am sure foam glue would work too.
If you don't have foam you can stuff the mattress cover with something like Poly-fill also.
Step 4: Attach Headboard and Footboard
Attach the Headboard and Footboard to the box ends with tape. I used gaff tape because I happen to have a lot of it, but it is a tad expensive usually. You can just use masking tape. I would not use duct tape, because it won't take paint well, unless you plan to spray paint it.
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
At this point I painted my base coat. Then it could dry while I made the mattress cover. I put several coats of paint on before I finished.
The finished paint job is shown. I used two colors.