Step 2: The foundation
In my case, i had a old fireplace that i used as a base for my oven. A friend made a iron reinforced concrete plate for me. It is 130cm/52inches square and 6cm/2.5inches thick weighting around 350kg. This was a heavy lift for 4 persons. I originally wanted it twice as thick and even larger...
If you don't have a fireplace to convert , you need to make your foundation from scratch.
Depending on where you live, you need to make it frost proof. How deep you need to dig for frost proofing, it's best to ask a local builder. I'd dig at least 50cm/20inches in a frost free zone. Then you can build up walls up to the oven floor height. Then fill it up with gravel, and compress it by jumping. Fill the last 10cm/4inches (minimum) with sand. That's where you will put your firebricks as your oven floor.
What bricks you use is up to you. If you have large stones around, use them. Kiko suggests to use "urbanite", that's scrapped concrete you can find at your local dump.