This is the basic rule for flanking; when you hit a creature that you flank with a non ranged attack your flanking ally gains advantage on one attack made against the same creature. Your ally must remain within five feet of the creature and the attack needs to be made within one round to gain this advantage.


Expert Flanker

Opportunity Retaliation: whenever a creature that you flank makes an attack against an ally you may use your reaction to make an attack against that creature.

Opportunist: When you successfully deal damage to a creature while you have advantage from flanking you add you proficiency bonus to damage.

Bully: when an ally makes a disarm, grapple, climb on top of, or shove to the side attack and you are within five feet of the target creature you may spend your reaction to grant advantage on the ability check.


Sword and Sorcery (5e) benjamin_shanafelt