One of the hardest things to do is to write the rules to a game you know inside and out.
Quotes about writing game rules
One exercise I remember from high school was to write instructions for how to make a peanut butter and jelly sandwich. Other students would then stand at the front of the class with bread, a knife, and jars of PB&J, then try to follow our written instructions as literally as possible, not making any assumptions. This was an activity we were all obviously familiar with, but it was amazing how poor most of the instructions were, omitting all kinds of details.
Another field that has the same problem is the writing of laws. National legislatures have lots of expert highly trained technical writers. How could there be any doubts about the meaning of laws? Yet trained experts (lawyers) can never agree about the meaning of any law. There is a need for courts and for a supreme court. So why can’t they just hire better technical writers?