If you are lucky enough to already have a hard solid surface like a patio area where you can build the oven then lucky you, because you can skip this stage of the build and move straight on to building the base.
Please Note:
The depth of the foundation and the thickness of the concrete will be dependant upon the type of soil on the site where you are going to build your oven. The steps below detail how I built my foundation which I felt was suitable for the earth in my garden. The earth was fairly solid and stable once the top soil had been removed. If your soil is very loose, sandy, or wet, then it would be best to dig deeper and to pour a concrete slab instead of using four individual slabs. If in doubt, then get some advice from a builder. The last thing you want is your oven to start to collapse due to weak foundations.