The place to start is Kiko Denzer's "Build Your Own Earth Oven".
I also made extensive use of Daniel Wing and Alan Scott's "The Bread Builders", which is a great resource even if you're baking bread in a regular electric oven.
Step 1: Bushwhacking, Lumberjacking and Limestoning
The most obvious place to put the oven was in line with our garden shed, but the whole area was a dense jungle. I spent a day with our Stihl brushcutter, with a saw blade in place of the string trimmer. That thing would be awesome for fighting zombies!
I had to chainsaw out a medium-sized box elder tree as well.
Under all that jungle, I discovered an improbable trove of cut limestone left over from when they built our house in 1973. Too good! I'll use that for the keystone and the stone veneer around the foundation.