He wrote a great blog post on this. You can just Google it. I'm just going to kind of touch on it but I would highly recommend reading the entire thing because it is fantastic.
A lot of the advice that I'll present today is actually synthesized fantastically in this book actually written by a YC founder called, "The Mom Test."
Marc Andreessen also has an amazing blog post about product-market fit, where he describes it as, "When the product is just being pulled out of you. When you no longer have to push the product on customers, they're just pulling it from you."
