I don't know how to use Ovalite.

Ovalite is a consumable item that you can set to a unit before embarking on a Quest. It increases the amount of JP earned when finishing the quest.
You can set 1 Ovalite per quest and multiple Ovalite when using Auto Repeat or Skip Tickets.
*When setting multiple types, the ones with the greatest effects will be used first.

You can set Ovalite by doing the following:

①On the quest Embark Screen, under Battle Settings, set Use JP Acquisition UP Item to ON.
②Tap the Embark button and the settings screen will be displayed.
③Set the Ovalite you wish to use here.