The easiest, way is to dilute drywall compound and put it into a a texture gun (sounds counter -intuitive but stay with me). The texture gun runs off of an air compressor. You can rent both from your local home center. spray a coat of the drywall compound onto the wall and imediately strike it off with a large taping knife (14 or 16 inches). This will give you a smooth finish. If there are small imperfections after it is dry, use a damp sponge and a light touch to smooth them out. Of course you will have to prime and re-paint the wall or door.