You will get the Corpse Camo after your Chicken Emblem playthrough. If you need information about the Chicken Emblem, see the bottom of this guide.
Equip your Corpse Camo by selecting OctoCamo [Manual] and picking Corpse Camo. Trigger an alert, and while near enemies, go prone and then hold the triangle button. This will fool the enemies into thinking you are dead, and the alert status will end.
Chicken Emblem requirements:
In a single playthrough, get over 150 alert phases, over 500 kills, over 50 continues, use over 50 recovery items and finish the game in over 35 hours. This takes over 35 hours to get.
You can get more information from an emblem guide. I used this emblem guide: http://www.playstationtrophies.org/forum/metal-gear-solid-4-...