When to get started and when to start stopping?

You have an idea for a business, or a hobby, or a project that you want to try. There’s something bouncing around in your head, desperate to get out, and you think you might know what shape it will take. If you’re like me, at this point, not much happens. I’ll sit and think, and think, and sit, and think some more. I think what I’m trying to do is figure everything out up front, before I actually do anything.

But here’s the thing, I can’t. You probably can’t either. So, if it’s not possible to figure everything out in advance, when do you start?

And what does it even mean to start?