Planning is an essential part of designing any game. You may think it isn't, but most games that aren't planned don't end up being finished. So, do it. Plan, and plan, and plan.

Make sure you have everything on paper. However, don't plan too far in. You don't want your game so complicated that it takes a Rocket Scientist to figure out the controls. You want your game simple. Make sure you have all that down.

Start with the Gameplay, Sketch up the interface, doodle the perspective, move onto how the AI works, jot down the difficulty, the heart of the gameplay, sketch the characters, show the controls, etc. Have just about every key feature.

However, I am not telling you to write out a walkthrough of the game. Just have the basics of the key features, and a few details. It also helps to draw what the characters, enemies, allies, etc look like. And to doodle out the level plans.

Also, don't forgot about sounds. They set the mood of the game. So, have what style they will be, what instruments, etc. You (or anyone else,) doesn't need to compose it yet, however.

Just have all that down on paper. For me, it usually takes 3-6 hours to do this process. However, I am a fast drawer. You don't need to rush through!~ I find that it is easier to focus on this in spare time after Exams. that way, the place is quiet, and you don't get tempted by the TV.

That's about all I can say about this subject, as (Mostly,) everyone has a different planning procedure.
Until next time, Happy planning!~