Would it be proper advice if it wasn’t the always present: “Just start”?

This is how I did it. I didn't even do a proper outline. I had been reading on Royal Road for a few months, and had a couple of ideas burbling in the back of my head. I eventually pulled some together and just started writing. 2k-ish word chapters, initially posting at three times a week (I've actually slowed down on the main story, but that's so I can add additional material and work on other ideas too).

I had a sketch of a campaign setting that I realized this story would attach to very well, which gave me half a dozen deities for my world and a reason to keep expanding.

I'm now past 200k words/over 100 chapters, and no reason to stop. I am enjoying everything about this process. I'd never have been able to do it if I had spent too much timing planning and prepping and outlining. But my story is very character-driven, instead of plot-driven.