Download this <a href="http://www.cutoutandkeep.net/photos/howto/papercraft/opensign/opensign.pdf">PDF</a> and print it out on to thick card.
Cut out the shape however you like. You could cut it out in a square of almost cut out each letter completely. I left a small margin around each of the letters and left a tab above the O and then N.
Punch a hole at the top edges on either side of the sign.
Cut a length of chain and slip either end through the two holes on the sign. Use a jump ring to attach the end of the chain to the side of the chain that is just above the hole. Hang up and you're all done.