Have a melee companion (or you) who does not have the Flanking Epic Talent attack an enemy. With the companion that has Flanking, attack the enemy from the opposite side. You will now attack them twice.
Flanking is an epic power. In battle, flanking will appear randomly if you approach an enemy. It will mean an extra hit before your normal hit, but it will not be every round. You have no power over when it will happen, so don't worry about it, its just another advantage you have over the enemy.