Ask HN: What is your process of validating an idea?

Hey guys, I've been working on building a site, areyouinterested.co, that helps people create easy landing pages to validate their ideas.

I'm trying to make it as good as possible and part of this is making sure that it fits into the general validation "flows" that people use.

So my question is, what process do you use to validate an idea? Do you build something first, or do you validate first? Do you try to build a community before or after you have an MVP?