Draw the shape of the base of the oven you want to create onto the floor where you will be installing it.
Mix your mortar according to the instruction on the package you purchased.
Lay a layer of bricks to fill the outline you have drawn, filling the spaces between the bricks with mortar. Cut any bricks that will be too long with the electric saw.
Add the bottom layer for the dome, adding mortar between the bricks. This layer should be built with the longest axis of the brick pointing toward the center of the oven and the second longest axis pointing toward the ceiling. This will ensure that you will have sufficient height for the interior of the over before you start curving in the rest of the dome.
Leave a space for the opening at the front. Add a brick on either side of this opening to be the base of the arch that will form the Roman arch for the door.
Create the next layer. This layer should be built with the longest axis of the brick pointing toward the center of the oven and the second longest axis parallel to the floor. To create the tilt in the brick required for beginning the curve of the dome, cut small triangular pieces of brick, and wedge them under the outside faces of bricks. Add another pair of bricks for the door arch.
Continue this process of adding layers that gradually curve in to form a dome. Let the bricks for the door arch remain vertical for most of the height of the door, and have a final sharp arch to close them.
Leave a small hole at the top of the dome for attaching ductwork for a chimney.