Summarize this article:

This mission is for Bonnie Clyde, and unlocks the first level of Dead Eye. You won't be able to use it at all until you complete this mission. Using Dead Eye drains the meter on the right side of the map. You can fill your meter by killing enemies or using certain items. Getting headshots on enemies will fill your meter faster. The meter doesn't need to be full to use Dead Eye, but you'll have less time available. You can use the following items to fill your meter:  Snake Oil Chewing Tobacco Moonshine (Gives unlimited meter for 10 seconds) Tonic (Made from harvesting plants after completing Legendary rank on the Survivalist challenges) Dead Eye is best used with a firearm, preferably one with several bullets. You can use Dead Eye with throwing weapons like the Throwing Knife, but you can only throw one per Dead Eye use. You cannot use throwing weapons and Dead Eye in multiplayer. You'll need to be aiming in order to trigger Dead Eye mode. Hold L2 or LT to aim your gun. You won't recover any of the meter you used. For the first level of Dead Eye mode, time will slow and you'll be able to line up and fire off shots. Press R2 or RT to fire shots while in Dead Eye mode. You can only shoot one bullet at a time using Dead Eye Level 1
Complete at least the second mission, "New Friends, Old Problems." Fill your Dead Eye meter. Equip a gun. Hold the Aim button to enter Aiming mode. Press R3 or RS again to cancel Dead Eye mode. Press the trigger to fire while in Dead Eye mode.