Let me be clear, their advice is great... for them. It might not work for you. Bear that in mind when listening to writers offer you their insights into writing.
There are as many ways to write as there are writers. My unsolicited advice? Find the way that works for you. Take the time to explore your own processes, and don't let anyone convince you that it's wrong. If it works for you, then it's right.
One of the more noxious pieces of advice I see flung around frequently is "write what you know." I'm going to go ahead and call bullshit on that - for the broad strokes. It's very true for the details. Sometimes. Let me explain.
If everyone only wrote what they knew, in the broad strokes of their stories, the only thing being produced would be memoirs. I mean, there's nothing wrong with memoirs, but I love my fiction. Just because you're not an expert in dragons, doesn't mean you can't write about dragons. One of the best things about fiction is that you get to just make shit up. Come up with a design, classification, and culture of dragons for yourself, and use that to write your story. It's going to be different from someone else's ideas about dragons, but the wonderful thing is, no one can fact check you. It's all made up.
When using the "write what you know" advice while building the bones of your story will guarantee that your story will be nothing but derivative and therefore dull. We've seen this in fantasy, where writers try to be the next Tolkien... by taking all their ideas from Tolkien. The elves, the dwarves, the race of men, blah blah blah blah. It's so boring. That's one of the reasons why I loved Erikson so much. It was new, fresh, fantasy without all the stuff we've read ten thousand times before.
I was terrified of it when writing Human. There's literally only so much new stuff you can do with vampire stories. Luckily, the reviews have all been pretty positive thus far.
So, in the broad strokes of story-telling, don't worry about writing what you know. Write what you don't know. Explore something new and unknown. That's what SFF writing is for.
That said, it is probably wise to use what you know in your writing. Erikson, for example, is an archaeologist and anthropologist. That shines through in his work The Malazan Book of the Fallen, wen he describes the appearance, clothing and weapons of the T'lan Imass. They are drawn straight from Homo Neanderthalensis. It was obvious to me when reading (prehistoric anthropology is an interest of mine). I had the opportunity to ask Mr. Erikson about it at Can-Con this year, and he confirmed it for me. I high-fived myself for guessing correctly... once he was out of sight.
The point I'm trying to make here, is that Erikson drew on what he knew in order to write something fresh and interesting. While actual Neanderthals did not participate in a communal ritual that turned them all into undead warriors capable of travelling great distances as dust on the wind, their appearance, clothing and weapons were all pulled from real life.
When writing Daughters of Britain, there was a perfect mix of what I knew and what I didn't. There was a lot of research that went into the book to make sure that I got the events and locations more or less correct. However, other than the names of some of the people involved, everything else was utterly made up. I drew on what I knew about "barbaric" and insular Europe following the Boudiccan Revolt. However, the people involved, who they really were, what they really thought, did and said, well, that was all made up. Do I know the names of Boudicca's two daughters? Nope. I made that up. Everything that Mederei goes through in trying to get back home... yeah, made all of that up. Civillis' son Adalbern... he's entirely fictitious. I don't even know if Civillis had any children. I mean, it's likely, but still.
I based the social structure of the Ragnar in Skylark on what I knew about communal insects and feudal societies, and also on speculations about gene-altering viruses. True story.
I used what I knew. I didn't write what I know. That's an important distinction, I think.
So, absorb everything you can. Read a tonne of reference books, academic papers, and first-hand accounts. Watch documentaries. Absorb it. And then use it touchstones in the unknown that you are exploring in your writing.
But don't feel like you have to limit yourself to only writing what you know. There is too much to explore for that nonsense.