The Design Document and Storyboards
Once you have a game idea, you need to get it on paper. Now, when I do a big game product, I require myself to make a real design document, but for little games, a few pages of details will do. Basically, a design document is a roadmap or outline of a game. It should have as many details about the game, the levels, and the gameplay as you can think of. This way you know what you're making and can follow some kind of plan. Otherwise, you will keep changing things and your game will end up being incoherent.
Usually, I like to write down a simple story to begin with, maybe a page or two that describes what the game is about. Who is the main character? What is the idea of the game? And last, how do you win the game? Then I decide on the hard-core details of the game—the levels and the gameplay—and outline them as much as possible. When I'm done, I can always add or delete things, but at least I have a working plan. If I think of 100 cool new ideas, I can always add them and not forget them.
Obviously, the amount of detail is up to you, but write something down. At least make some sketches! For example, maybe you don't even want a full design document and are more comfortable with some crude sketches of the levels and gameplay. Figure 1.3 is a sample storyboard that you might make for a game. Nothing complicated, just something to look at and work from.