Making it is a laborious process.
The acorns are first boiled (to leach the bitter tannins out of them), then roasted to develop their flavour, then chopped and then ground into a meal.
The ground nut meal is then dried in a slow oven for half an hour, and ground one or two more times.
Despite the multiple grindings, the flour will still be gritty.
When wet, the flour will be somewhat gelatinous and have a unappealing surface gleam.
Any recipes that do call for it are usually careful to specify that it should be processed first, e.g. “4 tablespoons leeched acorn meal.”
Store acorn flour in a dry, cool place.