https://www.themudhome.com/mudbuilding/how-to-make-a-cob-oven-and-avoid-the-common-pitfalls

https://theyearofmud.com/2009/09/12/outdoor-cob-pizza-oven/

There's a book too, if you want more detail, that's what I used when I first made mine like 12 years ago

https://permies.com/t/223763/Cob-Oven-bible-Build-Earth

The simplest explanation is you pile sand into a dome shape, then mix clay and sand and pile it on the sand, then mix clay sand and straw and use that for the outer shell. After the walls dry you can scoop out the inner sand dome.

I highly recommend renting a cement mixer, doing it with foot stamping takes a very long time.