This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for The Return of the Emperor side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 283,219 EXP, D Medal, Card 027
Where to start: West Area, as Goku.
To start this mission, you have to go to West City. Meet with Frieza - he will give you an additional story.
The only objective is to defeat Frieza. After that, you will finish the mission.