Collect materials.
Doodle!
Seal your blocks.
I got mine from a yard sale. Name brand or knockoff is totally fine. I didn't prep the blocks in any way. Just started doodling.
I picked up a 48-pack Pagather acrylic marker set from Amazon. These were great for the price. A lot of people use Posca paint markers, but those seemed too fancy to me to use on wood.
I didn't do anything to the blocks. No sanding. Just started doodling.
I used Rustoleum clear gloss spray paint to seal my blocks. I gave each block 2 light coats and let them dry 24 hours before playing. As long as you keep the coats very light, the blocks shouldn't stick when you play and it's been holding up nicely.