How does the Assassin's Creed Brotherhood recruit younger brothers?

Go to the white assassin's badge on the map, that is, the birdcage, and then click the League of Assassins to see if your little brother is full. At most, there are 12 towers, as many as the towers of the hostile forces (whose names have been forgotten) you burned. Without a younger brother, the plot is not here. I'll let you recruit a younger brother as soon as I arrive. If there is a younger brother, lock the enemy (press F) and then press T to summon the younger brother to assassinate. If you don't press t for a long time, it will be an arrow rain to kill Batman, but it will take at least six younger brothers to start.