If you're about to start writing you want to get the ideas down. If you stop and think about every detail you'll not be able to function. Turn off your internal critic. Set aside your self-critic and just get the idea down in any way you can. Come back after the fact to do the editing, add the detail, refine the writing.

Why do some people edit on the fly? They start with a kernel of an idea and try to force a beginning, middle, and end. They want to pull characters out of thin air and develop plot. They become so focused on drawing the map that they never actually go anywhere.

DON'T EDIT ON YOUR FIRST DRAFT!