You can equip Magi to your equipment by following these steps:
1. Tap the circle icon on the bottom right part of the screen and go to "Armory".
2. Choose a weapon or armor with the appropriate Magi slot. You can also press "Insert Magi".
3. Select the slot where you'll equip the Magi.
4. Select the Magi that you want to equip and then confirm.