Create a large compass. To create the large compass, nail the center of the piece of plywood to a wall or the desired location of the arch.
Attach a pencil onto the end of the piece of plywood using duct tape in a manner perpendicular to the edge of the piece of wood. Cut a piece of plywood into the same width of the desired arch.
Use the compass to draw the radius of the arch. Cut out the arc using a saw. If the incised wood is hollow inside, screw in some timber blocks to fill up the space. Do this for the entire arch.
Cut smaller timber blocks and screw or nail them inside the curved areas of the arch. They should be at a few inches apart from each other. The size of the blocks will depend on the size of the arch.
Close the opening of the arch by screwing on some long strips of plaster. The plaster strips should be a bit thinner than the opening of the arch.
Staple the arch bead starting from the bottom, and then around the arch. Once that is done, feel free to decorate it more as you wish.