Write, Don't Edit!
The most important rule of all. Everything else is secondary. Even "Show, don't tell".
It is the editor in your head you have to fight. He is nagging you: "You can't do that! What garbage have you written here? Are you serious? You will never be a good writer if you keep doing scribbling this nonsense!"
Well, you can do, you are serious and you scribble all the "nonsense" you like. You have to! Every idea that flashes through your head, write it down. Kick out your editor, kick him hard. The truth is, that more than 90% is garbage, what you are writing. But you will never write down the 10% which are really brilliant if you listen to your editor.
Have you ever started to write a letter or a paragraph of your book, found a spelling error, a grammar error, and stopped writing? Have corrected the error, thought about how you can formulate that better? And after you let your editor interrupt yourself, have you tried to finish your paragraph and did not know, what the hell you wanted to write? It was all gone. You were sitting there for twenty more minutes to make something up, when you knew what you wanted to write, when you started. But you listened to the bastard in your head, the editor, and now it's all gone. Silence him! Write, don't edit. You have plenty of time editing, when everything is written down.