The advantage of having a curved archway rather than a flat one is that you can use the weight of the bricks resting on each other to provide support (almost wedging themselves in place).
The first step here is to cement the first (ground) layer of bricks in place using a fine mix of cement and fine sand (a ratio of about 1:3 would be fine). At this stage, I would also cement in place the arch. Lay the cellophane sheet across the base layer and begin to fill with sand.
A sand mold of the inside of the oven is created so that the bricks can be supported on the sand whilst being cemented in place (prevents a collapse whilst the cement is wet!). The sand is wrapped in cellophane or plastic sheeting to prevent the sand from sticking to the inside of the oven where there may be some wet cement.
Begin layering up the bricks working up the side of the cellophane sheeting. At this stage, it is good to cement in place the chimney.
Having the chimney slightly higher than the door ensures that the smoke travels out of the chimney as opposed to just coming out of the front of the oven. Don’t worry about the cement looking a bit messy. This will be sorted in the next step.
Mix some more concrete up as in step 2. Liberally spread it over the entire outside of the pizza oven. Not only does this make the pizza oven look better, it provides more insulation which will keep your oven hotter for longer!
Once the concrete has dried (usually a couple of days), it is time to remove the supports for the archway. This will then provide access to the bag of sand that currently still occupies the inside of the oven. Carefully rip the side of the bag and begin digging out the sand.
Once the majority of the sand has been removed, the whole plastic sheeting can be pulled out bringing with it the remaining sand.
Thank you Phil Reilly for this wonderful tutorial.