In order to flank someone you need to work as a group. You devide your team into two groups. Give them names just for the fun of it. Snipers can run out and take their positions right away after knowing your teams plans. Snipers will most likely not take part in the flanking tactics. Though they can prevent your attacking group from even encountering the enemy.
Space yourselfs out a little bit. Advance useing the leapfrog tactic. One group covers the advancing persons butt. So that they can later help cover the back peoples advancement. When you have finally encountered the enemy and are ready to flank them. Make sure your cover is poping up down sideways from cover all the time. So that they don't know how many people are their. If they do a good job, your enemy won't have a clue if someone is missing from the pack. Alowing the flanking team to pick of the enemy from the side.
If you are trying to flank, quess what so is the other team. So those who are providing cover should watch out for those people. Your cover should have sharp eyes. Making sure to Keep the other team pined down.