And since it's not always easy to explain, here are a few other websites that try to explain how to keep score.

http://slocums.homestead.com/gamescore.html
http://www.bowling2u.com/trivia/game/scoring.asp
http://hubpages.com/hub/Bowling-How-to-Keep-Score
http://helpwithbowling.com/how-to-keep-score-in-bowling.php