Throughout playing through the All Ghillied Up mission, Captain MacMillan will lead you through the mission and give you orders on exactly what to do to avoid enemy detection. Listen to him and follow his actions closely and you shouldn't alert any enemies. If enemies become alerted, MacMillan will tell you that you've been spotted and the music will become more intense. If this happens, simply kill yourself or restart the checkpoint and try that section again. Once you've completed the mission without alerting any enemies, this trophy will be yours.