Requirements: Reach level 20+, finish training with Captain Ginyu, and finish the Frieza 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 Frieza if you see him.
Log out and back in to speed up the process.
Speak to him for a test where you'll win Death Beam.
Continue with the game or Parallel Quests and speak to them after each for more tests.
Second: Death Crasher.
Third: Emperor’s Sign
Final: Supernova and your trophy.
You are now free to change mentors.