Step 1: Find some stuff!
That is absolutely right. I had a hard time finding magnets, so give me a break. For the record, I found them in Ace Hardware, near the hammers (???).
Here's what you'll need:
Some posterboard (or some other kind of stiff backing)
A roll of gasket cork (or something similar)
Hot glue gun
Some markers or paint
As you may have noticed, I'm pretty ambiguous here in the materials. This stuff works, but you might find something better. As for the magnets, I found a roll of adhesive magnet, which works nicely.