From the Rise of the Dead bonfire in Shrine of Amana, ascend the stairs to find a locked door. Continue through the door to reach the Altar.
Conditions for unlocking the door leading to the Altar of Amana:
- Exhaust all of the dialogue for the first Milfanito encountered in the Shrine of Amana.
- Rescue the Milfanito in the small hut before the Demon of Song. Do not kill her.
- Be in hollow form.
- When playing in NG+, if you have previously gained access to the Altar of Amana, the Milfanito's do not need to be rescued again to unlock the door leading to the Altar.
- Dropping all of your Human Effigies or storing them in the Item Box will not activate the Altar.
- The Altar will restore your Humanity when you're hollow and haven't got any Human Effigies in your inventory or Item Box.