get (something) under (one's) belt
1. To get something to eat or drink. Once I get some proper food under my belt, I'll be ready to tackle any work you can throw at me! My father could become quite nasty whenever he got a bit of booze under his belt. That's why he gave up drinking.
2. To get some achievement, accomplishment, or record attributed to one's career or reputation. The legendary batter had gotten more than 2000 runs under his belt by the time he retired. As a pilot in training, it's always a relief to get your first real landing under your belt.
3. To learn or master something. Once he got the proper technique under his belt, Jacques had no problem finishing the project on his own.