Whatever gets it off your mind
I don't really think there is a universal answer to this question since the level of granularity needed to flesh out projects is so different from person to person. I remember hearing someone ask David Allen on a podcast one time how detailed projects need to be fleshed out and his answer was "Until it's off your mind." For some folks, this may not take a high degree of granularity. For some, it may take "pick up the pen" for them to gain traction on a certain project. And really, if that is what it takes, then great, pick up that pen and get to work! It doesn't mean you have to stop at picking up the pen, but it gets you gaining momentum.