|Interior of mouth arch|
Decide on the arc of the arch mouth ahead of time and build an arch form accordingly. Since I build brick ovens without forms, the arch form is the only one I need and I can reuse it in many ovens. The arch in my ovens are two different width because the face of the oven is wider than the mouth. This 'innovation' of mine gives full and easy access to all parts of the interior of the dome without changing the mouth-to-dome proportions.
Laying out the arch brick on a horizontal surface, accounting for minimal mortar joints lets you queue up before you mix the mortar.
Notice the buttresses on the outside of the front arch. Since the chimney will rest on top of this arch and the mouth arch, the sides need to have masonry containment.