Cut the masonite to the desired size using a table saw. Lay your canvas face down on a table, and lay your cut masonite on top. Use scissors or a rotary cutter to cut your canvas 2 inches larger than the masonite on all sides.
Remove the board. Paint it with a canvas glue paste such as Yes! or an acrylic medium. Cover the entire surface evenly.
Lay your canvas on the board as centered as possible. Press gently with your fingers across the entire surface to smooth wrinkles.
Place another piece of masonite or other flat board on top of the canvas and add weight, such as gallon jugs of water. Allow it to sit overnight to dry. Remove the weights.
Flip the canvas board over. Paint the leftover edges with glue and press them tightly to the back of the masonite. Weigh down the board and allow it to dry.
Paint the canvas with gesso to help the paint adhere. Allow the gesso to dry before applying paint.