an old door
chalkboard paint (this comes in spray cans, $7, or quarts, $10)
some old paper
Step 1: Step one: find a door to fit your bed
from wikipedia, standard american bed sizings:
Single/Twin: 39" x 79"
Double/Full: 54" x 75"
Queen: 60" x 80"
King: 76" x 80"
The door here is a standard-sized door, and the bed is a queen-sized bed. Where do you find doors? Good question! For me, this was pretty easy, as I used an old door that had come from my house (who wants to have a door to a hallway?) which was hanging out in the garage. But this is easy to do even without that. Check places like Habitat for Humanity's ReStore, where people donate their old household items like furniture, kitchen cabinets, windows, and doors, or even some thrift stores. Remember, even if you have an oddly-sized bed, there are tons of differently-sized doors which can fit it--you just may have to hunt more.