Beat the game after resurrecting Crono.
Beat the game after beating Dalton, and recovering the Epoch, but before resurrecting Crono.
There are two ways to receive this ending, on New Game +, at the millennial fair, go through the right part of the Telepod and then beat the game (you will be taken to the final boss battle), or defeat Lavos at the ocean palace.
Defeat Lavos and beat the game after returning from 600 AD for the first time (use the right side of the Telepod).
Use the bucket at the end of time when you first arrive there, i.e. just after you leave 2300 AD for the first time.
Fight Lavos after defeating Zombor on the Zenan bridge but before receiving the Hero Medal.
Beat the game after getting the Hero Medal but before going to 65,000,000 BC.
Defeat Lavos after receiving the Dreamstone but before talking to Frog when you have the repaired Masamune.
Complete the game after giving the completed Masamune to Frog but before going to confront Magus.
Crush Lavos after defeating Magus but before defeating Azala and the Black Tyrano in 65,000,000 BC.
Defeat the game after Azala’s demise but before battling Lavos in the ocean palace.
Beat the game after seeing Schala use her pendant to open the door to the throne room/Ocean Palace but BEFORE charging up Marle’s pendant at the Mammon Machine.