This part is pretty easy.
Figure out where you want the blackboard.
Sand/ scraper it smooth(ish). There is no need to get all Rainman obsessive about this part.
Tape off the borders.
Buy a can of blackboard paint. And a brush/ roller as needed. You CAN make your own paint, but it's SOOOO cheap; why bother?
Could have SWORN I took some pictures of this step, but apparently not.
If you need a detailed explanation, with pictures, for these steps, send me $99 and a Stamped, Self Addressed Envelops (SASE) and I'll make you your own little Instructable
... Or alternately, you could just ask ANYONE that has the first clue.