To use the sheep as a squad at the beginning of the "Operation Market" mission, walk straight through the field until you come upon some green bottles below some hay bales. Shoot the bottles in the following order: bottom right, top right, top left, bottom left. Do not shoot the bottle in the top middle. Turn around and walk back into the bush in front of the tree and press X. You should see confetti falling and can now use the sheep as a squad.