Re: How do you start designing a game?
1) What is the core of the game? Is there anything specific I want the player to feel/experience when making this? An easy way to decide the core is to make the elevator pitch: if you can sum up your game into one sentence, what would it be?
2) What mechanics support this core? What engine/program best supports these mechanics? What visual style supports this core? What are the most important sound effects+music that support this core?
If game has a story: what are the most crucial events and emotional beats I want the story to hit? How do I control branching? How do the mechanics/art/audio support this story? How does the story provide context for the mechanics/art/audio?
3) What is the scope? How long will I work on this game? What skills do I contribute, and what skills do I need from potential teammates?
4) Since I'm artist, sometimes I draw concept art of a still of the game and ask people that given this were on screen what they would do/try. This helps me figure out the most important things to communicate via UI and/or tutorial.
5) Make game. Bug test. Iterate. Playtest. Iterate. Playtest. Iterate. Repeat until investment spent on further fixes does not have enough bang for the buck.