Requirements: Reach level 30+ and finish half of the Android Saga.
After meeting these requirements, when you enter the city check all 3 main areas for mentors (There's only ever a maximum of two) and speak to Cell if you see him.
Log out and back in to speed up the process.
Speak to him for a test where you'll win All Clear.
Continue with the game or Parallel Quests and speak to them after each for more tests.
Second: Gravity Impact.
Third: Perfect Shot
Final: Perfect Kamehameha and your trophy.
You are now free to change mentors.