1. Write what you want, but make sure that it's right.
One thing you'll probably hear a lot is "Write what you know." Throw that out the window. Write what you want. My novel Class 5 is based in Arizona. I've driven through there a few times, but I live in Washington state. My personal experience with Arizona is peripheral at best. Why did I pick Arizona to write about? Dunno, to be honest. That's where the story told me it needed to be. I picked Arizona before I wrote about scorpions, or knowing the alien creature was susceptible to dehydration.
So, forget writing what you know, just start with writing, and if you have to, learn about it along the way. Just make sure you get as much right as you can.
2. The more eyes you can have on your work, the better.
Seriously. Get eyes on your work that aren't yours. Beta Readers, editors, proofreaders. It doesn't matter how good you think you are, you're going to screw up, and having other people read the work before it's actually published will ALWAYS help.
Beta readers will help you make sure your story is going in the right direction and point out where you go off the rails a bit. Most importantly though, they'll tell you if your story is entertaining! If your work isn't holding a reader's attention enough to get through the whole thing, that's a major problem, and without beta readers it's one that you likely won't find out until it's been published.
Likewise, find a good editor. This probably won't be cheap, but it's an indispensable part of writing, especially if you plan to self-publish. Languages can be complicated, and even if it's your birth language, you're going to get some of it wrong in writing. Punctuation, grammar, syntax, regional dialects, slang, all are going to make your life harder. Not to mention that after spending hours upon hours of staring at the words, you're not going to see them clearly. You're going to see what you meant more than what you put down. It doesn't even matter if you're an expert with a PhD in English Composition, you're going to need someone else to look it over. Don't be cheap. Pay up, and get a good editor to go through your story.
3. Luck plays a part.
Deny it all you want, but luck does play a part in your success as a writer. Luck isn't random though; luck is all those factors that you have zero control over. Things like how many other new books are released that day, how saturated the genre for your story is, how close your book's release is to people's paychecks, whether your book gets suggested among "if you liked THIS book, you may also like...". If you're trying to get an agent or in with a publisher, you may have the luck of being the seventh story of that type the agent has already been handed that day. Your script may happen to land in their inbox just as they accept their limit for the reading period.
But while luck will play a part in your writing career, you can work around it. Keep writing. Don't let a setback or rejection slip deter you. As the saying goes, "The harder I work, the luckier I get." The more you put your work out there, the more likely you are to drop it on someone's desk right when it's what they're looking for. It's the same for selling to an agent, to a publisher, or to a reader. That's the thing about luck, the more you play, the more the odds even out, and once that breaks even, it's all about you and how much work you've put in.
So, there you go, the three biggest points I would put into a "How-to-write" book. As all advice goes though, it's up to you to take it, or not. These are just the things I've learned in my time being a writer. Your mileage may vary.
~ Shaun