We already had some clay (it came out of the hole we dug and then filled with gravel). We lay a 10 x 20 tarp on the grass near the oven, and spread a five gallon bucket of clay on top of it. Then we sprinkled the clay with a little water from the hose, and started stomping.
It makes a huge mess, of course. It's fun, too, if you're into that kind of thing (and you must be, because why would you be reading this otherwise?). Also, if you're as out of shape as I am, it's exhausting and causes your legs to strongly consider seceding from the union.
Once the moistened clay is somewhat mushy, rather than mostly clods, it's time to add the sand. This time, instead of stomping directly on the mix, fold the tarp so it makes an envelope (with the mix inside), and then stomp on the tarp. Stomp for awhile, then open the tarp and admire your work. There will still be dry sand visible; try to distribute it evenly over the clay and fold the tarp over again. Stomp some more. Repeat until you don't see any more dry sand.
Congratulations -- you have adobe! (Take ibuprofen for your aching legs.)
Spread this batch of adobe across the top of the rock base. (When you built the rock base, you stopped adding the center fill a few inches before the top of the ring. Slap the adobe in there, and make the top as flat and smooth and level as you can.
This part of the process will take shockingly little time compared to how long it took to mix the adobe.