Ask HN: How do I make my first web app?

I can code in various languages and have worked on parts of larger (enterprise) web app developments, and some of my own tiny ones as well. I was wondering if some of the entrepreneurs on HN could really get into the hows of the technical and business aspects of their applications from day 1. i.e - how they went from Idea to real world product.

As we know - the idea itself is worth 0.