||Cut out the main section of the arch and glue it into the shape of a box. Note that the corner where the walls meet is marked on the tab with an asterisk.
Next cut out the inside of the arch. Roll the curved section over a round object to match the curvature of the arch. Before gluing it in place, test to make sure it fits correctly inside the walls of the main section. When you are ready to glue it in place, you can use a tweezers to push the inside piece perfectly into place before the glue sets.