So now that you've figured out what to write—and generally how it'll go—just write. Or, rather, write badly and create a first draft: The Ugly First Draft (TUFD) I talked about earlier.
TUFD isn't a pass you give yourself to produce substandard work. But it is a necessary part of the process of creating above standard work.
In Greek mythology, the goddess Athena was the favorite daughter of Zeus. She was born not in the usual way; rather, she emerged from his forehead fully grown and armed.
Much of writing paralysis is the result of expecting too much of ourselves the first time out. Sowing letters onto the blank page and expecting something strong and powerful and fully formed—the content version of Athena—to emerge is unrealistic. Unless you are some kind of deity, that's not going to happen.
Very often, the people you think of as good writers are terrible writers on their first drafts. But here's their secret: They are excellent editors of their own work.
So embrace The Ugly First Draft as necessary. As painful and depressing as it might be to write badly—at least you're writing, you're getting the mess out of your head and onto the screen or paper. Then, when you get back to it, you can start shaping it into something more respectable.
Recognize that brilliance—or anything close to it—comes on the rewrite. That implies that there is a rewrite, of course. And there should be.
As writing teacher Don Murray said, “The draft needs fixing, but first ...