Clay is one of Nature's many gifts! I could talk about this all day long!
To answer your immediate questions, I add clay to my bird food two ways: sprinkle it over dry feed, or add to feed mash (watered down feed). As far as the amount, for 2 hens, I add about a tsp to softened mash, you can just sprinkle on top or mix in. I generally leave it on top to make sure they ingest enough of it, as my hens do not always eat all their softened mash, and,... I don't leave mash out more than a few hours. I found leaving it out longer and it starts to ferment and kind of turn gross. I wouldn't want to eat it after a few hours, so don't expect my hens to. LOL For their dry feed, you can do the same thing, just sprinkle some on top.
I had a hen with bumblefoot. I soaked her foot and was able to remove the bumblefoot, then I created a stiff paste with the clay, packed that into the hole where the bumblefoot was, wrapped her foot with nonstick tape, and off she went. The bumblefoot never returned, she never had an infection. And the good thing about the clay is that if she pecked at her foot and ingested any of the clay, no damage done unlike chemical meds that should not be ingested. She actually never messed with the bandage either, she was probably so relieved to be rid of the bumblefoot. I cannot imagine walking on something like that!
This is the clay I use:
https://www.good-earthstore.com/ter...orillonite-clay-by-california-earth-minerals/