Macca was right

You don't know how it's going to turn out.

Good or bad, before and during writing, you just don't know how it's going to turn out. It's only afterwards that the real worrying begins: did I get it right? Did I get any of it right? Which bit is good? Which bit is not so good? Should it be longer or shorter? And so on...

Hmmm. How do you not worry about it beforehand?

I guess what the great Fab meant was that you shouldn't worry about your ability to do things if it's already been demonstrated that you can. Draw confidence from the fact that you've already done it, so you can be confident that you can do it again.

You could say, though, that this doesn't help you very much if you haven't written anything yet. But maybe it does. My first story was a short, and I didn't worry about it in the slightest. I thought about it, dived in, and came out the other side. Once again, it was only afterwards that I was able to tell whether it was any good or not. It was probably the only time I didn't worry about my actual writing.

Aha! That's it: think like a new writer!
What, every time? It's not possible. Once you've completed your first story, it's not possible.

Perhaps if we substitute 'worry' with 'doubt', it makes more sense. Or rather, makes it easier to understand. Don't doubt yourself. Think about your work. Do the research, the planning. Be positive. If you are any good, it will all turn out fine. But you'll only find that part out afterwards. And the knowledge gained will help you with your next work.

Hmmm. But Macca chose to say 'worry', and not 'doubt'. Then again, he is mostly musician - John was more of a poet. I wonder who wrote the words to 'Paperback Writer'?
Perhaps, in this context, worry and doubt mean the same thing.
I know, let's do both: don't worry and don't doubt! Acceptable overkill. Yeah, that's the answer.