This is an horno, or river rock-lined cooking pit.
Step one, clear the rock cover.
Step 2 dig out the dirt and coals from last baking session.
Step 3 light the fire!
And burn until there is a very large bed of coals to do the cooking.
Start in the afternoon and finish after dark. Set down a layer of greens (agave leaves work fine) before wrapping the agave in wet burlap and setting on the bed of coals.