Serious question: Do I or other Creators really come off as narcissistic when we post things? I personally don't think I'm better than anyone else here simply because I'm a volunteer contributor to a porn game. If I'm giving off that vibe, I'd really like to know so I could try to adjust my behavior.
Anyways, onto the main point...
Writing, like any skill, is honed through practice. A lot of practice. I've been writing things since I was in middle school. I didn't really buckle down and start taking it seriously until my late teens, but I've been doing creative writing in some form or another for more than half my life. That's really the only answer I can give as to "how I did it." That and reading. Reading improves your writing improves your reading, etc. Being able to understand the language helps you to write the language and vice versa.
Education-wise, I just took your standard fare of elementary, middle, and high school English classes, and then I did a little bit of college-level English until I dropped out. So it's not like I'm an excessively over-educated academic or anything. I don't know about any of the other writers here, but I know from my own experience that you don't need an advanced education to become a competent writer.
Regarding erotica itself... Uh... It helps to be a huge pervert? I don't know. I just kind of started writing it late in high school. When I was a budding pervert. I can promise you my early attempts at writing smut were God-awful. If I still knew where they were, I'd probably read them if I ever needed to make myself physically ill for any reason. But I kept practicing and I eventually became less shitty.
There's been requests in the past for a "Writing Guide" post to be thrown together, but no one has really been super inclined to do that. Both because it would be a huge undertaking and because I don't think anyone here is much of a teacher. Or at least doesn't want to be a teacher to FenForums. Being able to educate is its own skill. Simply being good at something does not mean you can teach it to others, unfortunately.