I had some spare frames that had come down when we changed things around in our store, they'd been sitting around for ages just collecting dust so I decided to put them to good use.
Step 1: Remove the backing board
Most frames will come with a backing board, hopefully think enough to withstand the pressure of children smacking it with chalk stick. If you don't think it's robust enough it'll be short work to use the existing board as a template and cut a piece ply-wood as a replacement.
Remove the backing board and set it aside to paint.
Step 2: Paint the backing board
I got some cheap spray-on undercoat and gave the backing board a pretty thick coat then hit it with a can of spray-on black board paint. Follow the instructions on the cans regarding how quickly you can re-coat etc.