One of my goals for O's birthday party was to make a huge outdoor chalkboard. I barely made it...I was still painting it at around 12:00, and the party started at 3pm. It was fairly easy to make.
1st- buy 1 piece of treated plywood and cut it to the desired size
2nd- paint the plywood with primer. You may want to sand it first. I painted ours with 2 coats of primer, and it still had rough spots that I filled in outdoor wood filler. Then I had to wait for the wood filler to dry, sand it, then paint another coat of primer. After 3 coats of primer, I painted the board with chalk paint
3rd- I taped off a border with masking tape, and then painted it with 2 coats of chalkboard paint.
4th- After the chalkboard paint dried, I pulled off the masking tape and painted the border red. I painted 2 or 3 coats on the border.
The kids loved drawing on the chalkboard at O's party, and its also lots of fun erasing the chalk with water using paint brushes.