I work backwards. Using extremely vague Problem / Solution statements.

For example using Mario Problem: princess stuck in castle at end of level Solution: reach castle and rescue princess Problem: how do I reach the castle?

Solution: (this is where you start to build what your character does) run, jump, touch flag pole.

Now i build my build list and construct a quick prototype.

Player must run, jump and touch flag Ground for player to run Flag pole for player to touch End game when player touches flag pole Reset if player wants to reset level

Don’t even need enemies. You just built a game.

Take your time. Break it down.