Do not try to be a fortune teller by planning to the smallest detail. Finding a good balance is the true art.
Haha, yah. My previous projects have all been modifications and had required planning really (or I just didn't choose to plan them on paper). More or less 1 system being worked on at a time, and a matter of just implementing and testing it, then rewriting it if it wasn't that good. I've learned a lot from that, but started realizing I should probably plan things out as I move into writing a whole game from scratch.
So far, I only have a few things written down on how game Entities and Worlds are going to be handled and what is the absolute necessary information they need to hold. My biggest issue is learning the rendering, I understand the concepts but it can be a bit difficult translating the math knowledge back down into code when there are so many varied examples on the net (especially with the large differences between GL11 and GL30)