To get this you must talk to the Bell Keeper at the entrance of either Belfry Luna or Belfry Sol.
Belfry Luna is accessed through the Lost Bastille. Once you have reached the Servant's Quater bonfire (thanks vSully) there will be a hole and a ladder going down. Drop down and activate the Pharros contraption. This will reveal a hidden wall, go through, open the door, and the Bell Keeper will be sitting on your right as you enter. Talk to him to join the covenant.
Belfry Sol is accessed through the Iron Keep. After defeating/skipping Smelter Demon, continue onwards across a bridge in a circular room (a turtle guy tries/succeeds at breaking the bridge). After this room on your left there is a room. Enter it and active there Pharros contraption. This will reveal a hidden wall. Go up the revealed ladder, and at the top the Bell Keeper will be sitting just before the fog gate. Talk to him to join the covenant.
Note: you do not need to join for the achievement to pop, you must simply reach the decision window (join: yes/no).