What you need: White grout, Acrylic paint, wood (or whatever surface you want to paint), paintbrush and measuring stuff.
Make a mixture of the grout and acrylic paint in a ration of 1:8. I used 1 tsp grout to 40ml acrylic paint, and I had loads left over!
Paint your surface. I did a few layers all in one go and smoothing it out as much as possible with the last layer. Using a light sandpaper can help to smooth the board a bit, then go over it lightly with chalk.