Write a spec... What sage startup advice by YC lol. Who would have thought.

Seriously though check out Shape Up: basecamp.com/shapeup

It’s a whole book on how 37signals makes the sausage. Goes over how they spec out a feature, decide on what should be built, etc. They’re a team so not everything applies to individuals but there’s a lot of useful ideas. They give concrete examples and case studies. Including specific examples of specs.