This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for One Cool Dude side quest.
Time frame: From Evil Emperor Frieza - Episode 8: Intermission - Episode 8: Intermission
Reward: 38,616 EXP, D Medal, Awesome Book for Grown-Ups
Where to start: Western area, as Gohan.
You will start the quest by talking to Puar at the Capsule Corp.
Go to the other end of the city and observe Yamcha.
He will be attacked by opponents. Focus the enemies' attention on yourself and defeat them.
After that, you will complete the mission.