I used Circa 1850 to remove the varnish and a layer of paint, then sanded it down. Even though some of the paint remained, I knew the chalk board paint would cover it so I didn't worry about removing it all.
Since I wanted to try my spray on chalk board paint, I masked off the parts of the board I wanted to keep natural.
Then I painted it using two coats of Rustoleum spray paint. What a quick project! It dried in 20 minutes, and after an hour I removed the tape, primed it with chalk and wiped it down.
A loop of rope through the top, plus some to hold a piece of chalk (with a bit of E6000 glue to hold it on the back) and I was done!