making the inner arch for a barrel vault

[chickade]

Hello. We are working on our last dome arch and need to figure out the inner arch for the vault. We are using WJW's pictures. How do we cut the bricks to make the inner arch? Is it just the middle brick that needs to be narrower on the bottom than the top? Any tips for setting up the arch? Do we build a form for it? Thanks, Sarah and Josh

The attached picture is one of WJW's and is one of the main ones we are using to figure out the front of the oven.