If you haven't done so already, you need to collect one of these Titan loadouts for BT:
from Mission 3: Into the Abyss
- Chapter 3
from Mission 6: Trial By Fire
Next you must equip either the Brute or Northstar loadout by pressing
. Then you need to damage Titans to charge the Flight Core
up to 100%. Finally you must kill a Titan with your Flight Core, which is activated by pressing
. Unless you are playing on Easy, you may need to damage the Titan first.