When I created my design document for Friday the 2013th, it was quite easy, I only needed to list everything I wanted to put in the game - loosely based on the original - and then go point by point and implement these mechanics.
100 Reasons To Break Down (my current project) is a MetroidVania that I would call ambitious in some ways. I was stunned at first by how big some part of making a game from scratch can be. I didn't really have a design doc for Brooman Potionmeister or for Grid Drill, I just made it up as I went, but for 100RTBD I want to plan everything beforehand and then work on implementing it.
It's the same thing, really, plan things beforehand, make sure the whole game is more or less complete before you even start coding and then try to do all of the things you planned for. But right now, the planning stage is really a daunting task, I'm currently making the map and for a project of this scope, it's a big part of the work.
Currently working on: Designing the map
Time spent on the project so far: 8 hours