In Tabletop Game Design for Video Game Designers, I recommend taking the time to make a formal, comparative study of how professional game publishers write their rules before attempting to write one yourself. Doing so helps you understand the level of detail required to properly explain game mechanics and gives you a toolbox of techniques for doing so.
Below are some links to tabletop game rulebooks. If you are inclined, you might try reading a few to see how the information is presented. As you read through a rulebook, you might want to pay attention to the special terminology it employs, how and where does the rulebook present the definitions of its terms (if at all), and the use of graphics for explaining and clarifying rules.