Advice for aspiring writers

A couple of days ago, someone asked me via PM if I could give them some advice on starting a professional writing career (they were interested in writing SFF and romance). I'm probably not the best person to ask given the current state of my professional writing career -- but I went and wrote out a ginormous infodump in a PM, and then I decided to repost it here, because it might be useful to someone else, I guess?

Plus, you guys can tell me if I'm way off base with any of this. Or if anyone has additional suggestions, tell me, and I can edit them in!

( Advice for aspiring writers )

So there it is ... maybe someone will find this handy, or maybe you guys have some better suggestions and/or would like to point out places where I'm way off base here?