This page of the Biomutant guide contains a walkthrough of the Find the Twing-Twangs side quest. Here we've described everything you need to do to complete the task and earn rewards.
Starting the quest
How to start: Finding the guitar.
Reward: 100 XP.
To start the quest, you must come across one of the guitars. They are scattered across different parts of the game world.
Location: Leftovers 6E.
There is a guitar in the building that you must approach. You will tune the guitar by adjusting the keys accordingly so that the colors match.