How did Gwyn create the Darksign and link the First Flame?

Hi there.

I'm a long time Souls-fan who occasionally delve into the lore of the Souls-games.
It's as much fun as it is frustrating.

There's a question that popped into my head recently, which I don't believe I've read anyone else asking:

How did Gwyn create the Darksign and link the First Flame?

We all know that he did it, but how?

How did he create the Darksign, and link Humanity to the First Flame?

How did he link the First Flame by casting himself into it?

The question of how he linked Humanity to the First Flame is really interesting. What method did he use?

Did he manage to create the Darksign and link the flame by performing some ultra mega miracle through the usage of faith, or did he utilize some ancient ritual written down on some parchment, kind of like what Marika does to achieve godhod in Elden Ring?

Please give me your two cents!