You control a nontoken Zombie or a nontoken creature with changeling with mana value one or less.
Liliana has at least three loyalty counters on it.
Steps
Activate Liliana's third loyalty ability by removing three loyalty counters from it, allowing you to cast Zombie spells from your graveyard this turn.
Activate Phyrexian Altar by sacrificing a nontoken Zombie or nontoken creature with changeling with mana value one or less, adding one mana of any color.
Cast the Zombie from your graveyard by paying its mana cost.