Cut your Masonite board down to size if necessary. Make you guide lines with a T-square or ruler and pencil. Then cut along those lines with a circular saw. Sand the cut edge lightly to remove any shavings. If you do not own a circular saw, the lumber yard where you purchased the Masonite will likely offer free cutting.
Sand the smooth side of your Masonite board just enough to roughen it. This will help the gesso adhere. Wipe the sanded surface down with a tack cloth to remove any sanding dust.
Apply one layer of gesso to the Masonite. To achieve a smooth surface, paint a thick layer of the gesso on with a paint brush then roll it smooth with a small roller.
Allow the gesso to dry for the manufacturer's recommended amount of time.
Paint the back of the Masonite following Steps 3 and 4. This layer of gesso on the back of the Masonite will keep the board from warping.