It’s very simple, just go pro.
If you’re not good enough...STFU and get your degree for free while you play.
NBA G League players generally do not sign contracts with the individual teams, but with the league itself. G League team rosters consist of a total of 12 players, 10 (or fewer) being G League players and two (or more) NBA players. The rosters are made up in a number of ways: the previous years’ players, players taken in the G League draft, allocation players (meaning players who are assigned to a team with which they have a local connection, such as a University of Texas player being assigned to the Austin Spurs) and NBA team assignments. Each team also has local tryouts, and one player from the tryouts is assigned to the team.
The minimum age to play in the G League is 18, unlike the NBA which requires players to be 19 years old and one year out of high school in order to sign an NBA contract or be eligible for the draft.