Seal any cracks or holes in the door with wood filler and a putty knife. Spread the wood filler into the holes, and clear off the excess with the knife. Allow the wood filler to dry for the amount of time specified on the jar.
Sand the wood filler smooth with 180-grit sandpaper. While you use the sandpaper, look for any rough spots in the door, and sand them, too.
Place painter's tape along any parts of the door you do not wish to paint. If the door has trim or a frame, for example, tape along the edges to protect them.
Stir a can of chalkboard paint, and fill a paint tray with it. Push a paint roller through the tray until the roller is saturated. Paint the door with long, smooth strokes, and allow the paint to dry. Use a paintbrush to touch up any areas of the door, such as the edges, that you couldn't reach with the roller. Allow the paint to dry for the amount of time specified on the can. Apply additional coats as needed.