Author-illustrator Tomie dePaola said, "“Sometimes I still get up in the morning, I face the blank piece of paper and my brushes are clean and ready to go, and I panic. I know I’m going to make a mistake. So I get that over with, rip it up, and then get on with it.”
There's tip number one: Start by writing an intentionally bad sentence. Make it the worst sentence you can think of to express the thought. Then throw that one away and get to work.
World-famous author Judy Blume said, "“Tomie talked about courage. I was far from a courageous or brave child. I was timid, shy, quiet – except inside my head. Inside my head I was this other person. I was Madeline! I was brave! When it came to my writing, I never thought twice about it: I was brave in my writing in a way that I wasn’t in my life.”
She added tip number two: “Do not let anyone discourage you. If they try, get angry, not depressed.” (To which I would add, be sure you get angry at them, not at yourself.)