MathematicalThinking

Ask yourself questions like: How many players will there be? How long should the game be? What choices will the player make, and when will they make them? How will the player make these choices? How will one player's choice impact the other players? How will the players interact with each other? Are there any choices that can be made by one player, but not by the others? How does the game progress? Is it strictly turn-based, or is it in rounds with phases? What actions will the player be able to take? How will the outcome of an action be determined? What is the player's goal? How can the player win?

Once you've settled on your theme, you need to determine what your game is about, and how it's played. Start by writing down everything you can think of relating to the game, and putting every idea you have onto paper.