How do you know when to stop planning and start building your SaaS?

I keep seeing the same pattern in myself and in other builders. You have an idea and then you freeze between phases, is this worth validating, what's actually the MVP, how do you get first users without even launching yet.

There's tons of info out there, reddit, youtube, ai chats, but it's all scattered so it's easy to feel like you're doing stuff without actually moving forward.

I'm mapping my own process into something clearer, idea, validation, mvp, first users, launch, with a checkpoint at each step to decide if it's actually worth continuing. Turned it into a small project called SaaS Launch OS, still on a waitlist (saaslaunchblueprint.com) if anyone's curious.

What I actually want to know is how do you personally decide when to stop planning and just start building. Do you follow a checklist, go by gut feeling, use some framework. Curious what's worked or not worked for other people here.