Step 1: Proper Prep work makes the project (the seven P's are Proper Prior Planning Prevents...)
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.
Step 2: Paint it
Let it dry before you pull off the tape.
RETAPE to mask off the width of the alternately colored border, if you want one. Paint the border and let the paint dry before you remove the tape. I put one on the "inside April" blackboard and didn't bother for the one on the outside portch.
Step 3: Be patient
Get some chalk and turn the kids loose.
Try to keep the kids from seeing if they can use their crayons on the blackboard. I reccomend confiscating ALL crayons and letting them suffer using just chalk.