How it works
Let someone else try your project. After you have built it for a long time, your own eyes stop noticing its gaps.
Keep quiet and watch. Telling hides the problems, and watching shows them.
Write down what they do, and what happened, not why. Then fix the biggest problem first.
Let someone try it once more, because every fix needs a second try-out. A player is never at fault: the project had a gap.