No pre-prep is needed for this glitch as it works everytime, as this was overlooked by the developers in Rockstar. When equipped with the fire bottle, you are still allowed to go into dead eye. Although the aiming reticule will be gone, it is still "present" during dead eye mode, in the middle of the screen. Therefore, you can still "paint" a cross on your enemies, assuming if you have the reticule on any part of their body. Once marked, Marston will throw the fire bottle. When thrown, the fire bottle defies gravity and actually locks onto the enemy you selected and acts like a flaming homing missile. This is great for people speeding away on horses/carts. The bottle will keep flying until it hits the target and explodes in a fireball.