under one's belt
Experienced or achieved, as in Once a medical student has anatomy under her belt, she'll have much less to memorize. This metaphoric expression likens food that has been consumed to an experience that has been digested. [Colloquial; first half of 1800s]
from The American Heritage® Dictionary of Idioms
have sth under your belt
to have learned or succeeded in something which might be a benefit in the future:
Basic computer skills are a good thing to have under your belt.
(from Cambridge Advanced Learner's Dictionary)