Ask HN: How-to make solid profits from niche websites?

I often see newspaper articles and current affairs television programs, which tell the "success stories" of niche website and/or blog operators who earn six- to seven-figures each year directly from their sites.

I (and I think many HN regulars) would love to do something like this full-time and have some good ideas for sites but don't know where to start...

O learned HN gurus, if someone tasked you with starting a successful niche website from scratch, what would you do?

Any advice or links to relevant resources is greatly appreciated. Thanks.

P.S. For the purposes of discussion I define successful as: good enough to provide you $100k+ annually for working 40hrs per week on average.