At the beginning of the Operation Market mission, go forward through the field until you find some green bottles on and below some hay bales. Note: This is before the intermission sequence with Nicolas is triggered. Shoot and break the bottles in the correct order then turn around and go back in the direction you came from. Walk inside this bush and press Square. A sound and falling confetti will confirm that the Sheep Squad has been unlocked. You can now use the nearby sheep as an additional squad.