“I don’t even know where to begin,” is a place to begin.
Just start there!
Write, “I don’t know where to begin, because I’m concerned about how it’ll turn out. Because…”
Or, “I don’t know where to begin, and I feel…”
Or, “Today, I started writing and then I stopped. I stopped because…”
Remember, you don’t think to write, you write to think!
If you’re wondering, “I don’t even know how to do that,” that’s a great starting point to imitate a classic.