Here are the steps according to this Wikia, I haven't done these endings in ages but all of this sounds correct.
Ending 1e: Revenge for Cyrus
Method: When Frog and Magus face each other on the North Cape, either
fight and kill him or spare Magus' life
In the canon ending, Frog spares Magus' life, as killing him will not
return Cyrus. Magus joins the group for the rest of the adventure,
leaving at the end to search for his sister. However, if you choose to
fight Magus in a one-on-one battle and defeat him, Magus' spell that
transformed him into a half-human / half-fiend will wear off, but not
until the end of the game.
After defeating Lavos, Magus' magic wears off, and Frog turns back
into Glenn, who is a tall, muscular man. This is because he aged ten
years while in his fiendish form. He bids farewell to the others,
preparing to return to the Middle Ages. Lucca and Marle use the
opportunity to remark on how handsome Glenn truly is, wishing that he
could stay around longer.
This does not change the ending of the game.
For a complete list (of all the endings), check out the source below.