Press "Pause" and select the "Options" menu. Scroll down to "Cheats" and press "Y". The keyboard will pop up and you'll be able to enter your code and unlock the cheat you wish to use. Cheats will be logged into your "Cheats" tab and can be toggled "On" and "Off" at will. Codes must be entered identically as listed. All punctuation must be done correctly.

Password	Effect
Abundance is everywhere	Infinite Ammo.
Beast and Man together	Spawns a random Horse nearby.
He gives strength to the weak	Invincibility.
I don't understand imnfinity	Infinite Deadeye.
I'm an American. I need guns	Unlocks: Buffalo Rifle, Schofield Revolver, Winchester Repeater, Fire Bottle, Semi-Auto Pistol, Sawed-off Shotgun.
I'm drunk as a skunk and twice as smelly	Makes you constantly drunk.
It's my constitutional right	Unlocks: Volcanic Pistol, Springfield Rifle and Double-barreled Shotgun.
Make hay while the Sun shines	Infinite Horse Stamina.
The old ways is the best ways	Adds Sepia Filter.
You got yourself a fine pair of eyes	Unlock All Areas.