In Learning Video Game Design on the Tabletop, 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.
A comment on this post pointed to a collection of rulebooks from games published by Z-Man Games. Most game publishers have their rulebooks available on their websites, so it is not hard to find more examples than the ones I link to above.
Matteo Menapace says
You can find loads of downloadable rulebooks here 🙂 http://www.zmangames.com/downloadable-files.html