This Seeking Tech tutorial will explain how you can change the turn order in Clair Obscur: Expedition 33.
You can use the following video as a visual aide:
1. Open the menu, and then remove all characters from party aside from the one you want to play first in the turn order.
2. Add the character you want to play second in the turn order to the party.
3. Add the character you want to play third in the turn order to the party.
4. If you are using First Strike, equip the Lumina for all active party members.
5. Run into enemies to start the battle. Attacking enemies on the field may change the turn order.