down cold
Learned, mastered, or understood perfectly, to the point of requiring little or no focus to do, recall, or accomplish.
Make sure you practice these equations until you have them down cold.
I always get my routine down cold so there won't be any room for error during the performance.

have (something) down cold
To learn, master, or understand something perfectly, to the point of requiring little or no focus to do, recall, or accomplish it.
Make sure you practice these equations until you have them down cold.
I always get my routine down cold so there won't be any room for error during the performance.

@IdiomsLand