When you’ve gathered up the right materials you can Tune-Up a Toon by going to your Toon collection and then tapping on a Toon of your choice. Tapping the “Tune-Up” button will let you see if you have the materials necessary to Tune-Up the Toon.

Tuning-Up a Toon unlocks large boosts to stats or sometimes entire skills that the Toon didn't previously have access to!