Measure the width and height where the arch will be located with the tape measure. Make a note of the measurements. Transfer the measurements to a 3/4-inch sheet of plywood and cut the plywood with the circular saw.
Measure the width of the plywood. Make a mark on the edge with the pencil to indicate center. Drive a nail into the edge of the plywood on this mark with the hammer.
Tie one end of a piece of string to the pencil. Place the pencil on the edge of the plywood to the left of the nail. Tie the other end of the string to the nail.
Pull the string taught and draw the outline of the arch on the plywood. Cut along the outline by using a jigsaw.
Repeat this process to cut another piece of plywood to the shape of an arch.
Subtract 1 1/2-inch from the measurement for the width. Transfer this length to a 2-by-4 board and cut two pieces with the circular saw.
Set the two cut pieces of arch upright. Place the two cut pieces of 2-by-4 on their short side in-between the bottom edge of the plywood. Secure the plywood to the 2-by-4 boards with nails.
Measure the length of the arch with the paper tape measure. Transfer the measurement to the 1/4-inch luan. Cut the luan with the circular saw.
Secure one end of the luan to a 2-by-4 board on the arch frame. Bend the luan along the top of the arch and secure it to the other 2-by-4 board.
Construct a temporary support for the frame with the 4-by-4 posts. Place the frame of the arch on top of the 4-by-4 supports.
Apply mortar to one side of a brick and set in place on one side of the arch. Apply mortar to one side of another brick. Set this brick in place next to the fist. Tap on the brick with the butt of the trowel. Repeat this step to lay the remaining bricks halfway along the arch.
Begin laying brick on the opposite side of the arch. Repeat this step until you reach the halfway point.
Apply mortar to each side of the keystone and then set it in place.