This is something that should be obvious, yet despite everything I’ve written about beginners designing games I have not said it explicitly. And I know from my teaching experience that to many people it isn’t obvious. It is especially important for people who want to design video games rather than tabletop games.
When you set out to design a game it’s important to know what you want that game to do, what the impact will be on the players. But it’s much more important to know how the game is going to do that.