Question about Existentially Prolong

The requirement for Existentially Prolong is "Complete your first manual Eternity with at least 1e400 IP"

Does this mean i can go into eternity, then get 1e400 or do i need to have 1e400 before i eternity for the first time? I saw others say this takes 1-4 hours. I'm on my third reality and have 3 power glyphs equipped and my IP gain seems to be stuck. I want to be sure whether i need to do this before i eternity or i'm supposed to eternity first, then i get the next one with 1e400.