Right after you defeat Quleag and ring the second Bell of Awakening head down the stairs to the right of the lever and you will be in a circular room one wall seems slightly flat compared to the rest attack it to make it vanish. Talk to the minion in your way and tell him "Yes" when he asks if your the new servant, if you say no you'll have to kill him locking his available pyromancy spell until the next playthrough.
Talk to the giant white spider and say "Join Covenant" you don't have to actually click yes after that to get the trophy just wait a few seconds.
:Do note while in the covenant you need feed her humanity until you are a Chaos Servant+2 in order to obtain two more pyromancy spells for the "get all pryomancies" trophy, you must also be a CS+2 After obtaining the Lordvassal in order to have Solaire assist you during the final boss fight.
Heres a visual representation I take No credit for this video but note he says things that may be offensive to some viewers so if you are of the sensitive type I recommend muting his dialogue. Please leave a like on his Yotube page if you wish to support this video.