The NPC for this covenant is Alvina, the great feline.
She can be found in the Darkroot Garden beyond the door locked by the Crest of Artorias. You will be attacked by several NPC members of the covenant as you make your way through this area.
Talk to Alvina and she will ask you some questions. Answer yes to her twice and you will be invited to join the covenant, unlocking the achievement.
As always, refer to http://darksouls.wikidot.com
for specific information relating to items or locations.