Instructions
Load up your "Metal Gear Solid: Peace Walker" save file.
Enter the Missions menu.
Complete all available story missions until you reach chapter 5. You cannot unlock the bandana until you reach chapter 5.
Complete Extra OPS missions 113, 115 and 117. These missions require you to beat upgraded versions of the final bosses from the earlier story chapters. Once you have completed these three missions, Extra OPS mission 119 will become available.
Complete "Extra OPS" mission 119 with an "S" rank to unlock the bandana. It may take a few tries to reach the "S" ranking. Rankings are based on a wide range of factors. Some important factors include the amount of time spent beating the mission, the amount of damage taken during the mission, the amount of times enemy units spot the player and the choice of weapons during the mission. Experiment with different weapons and tactics to find a method that fits your playstyle.