Take a piece of cardboard and paint it with black acrylic paint. ( the cardboard must be the size and shape that you want your black board to be)
Now that it is completely painted , let it dry for about 6 min. Remember not to touch it when it is not completely dry! And if you get paint on your clothes , go wash up right away because after it dry's it won't come of. ( don't worry if you get your hands stained )
Finally !! You have your verry on black board. If you want to make it look fancy , you can put your black board in a frame and take out the glass.