Wiktionary
a. (context idiomatic English) Thoroughly practiced, rehearsed, or understood; mastered.
WordNet
Usage examples of "down pat".
He had noted the little kowtow and salute which the Elders used, and he had it down pat.
So you realize now that the Dreeghs had you down pat, that all your mighty pretension was simply your Klugg mind demanding pretentious compensation for a dreary, lonely life.
Once he got the management of Grantville MaidenFresh Laundries down pat, they could go to Teacher Muselius's catechism review classes on Saturday afternoons and he could go through it all again, more slowly.
The advertising boys had got it down pat -- even her best friend wouldn't tell her!
However, in the end, it was decided that this was not sufficient grounds to exclude him from membership, especially since he had the secret of the Philosopher's Stone down pat, and knew all the other requisite spells as well.