Lets skip past normal everyday mortar as you can find that info everywhere. What i'm talking about is 'refactory mortar" , this mortar is specifically designed for heat.
I'll give you the recipe a little later.
The secret and problem is that you want to use as little mortar as possible as the mortar may break down over time and you'll end up with mortar in your pizza ! crunch!!!!!!
The object is to create an arch at teh top of your oven that is almost self supporting, kind of like a dam wall arch, the weight above pushes down and to the sides, the mortar is just filling the gaps as your bricks are rectangles.
As I said earlier you can buy specifically cut angle bricks but you would be very lucky to find enough leftovers on a building site.
How I have got around using alot of mortar is to put in 1 angled brick at the start of the arch, I found a few but do need to pay to get some more cut. By placing this angle brick allows for the bricks to have a shallower angle and thus less gaps and thus less mortar.
this is a good link with lots of good links: http://www.ovencrafters.net/
