Every pruning job is different, but there are some basic rules. First, wait until late winter, so you can see what you are doing - particularly with a japanese maple, the form with or without leaves is important. Second, remove branches so that you are cutting them back to major branch angles and not just cutting anywhere. Doing this can reduce the height and width while maintaining the shape of the tree. Third, Remove any suckers, water sprouts, and branches that cross or rub. Try not to remove more than 1/5th of the crown, and if you need to remove more than that, do a little this winter, and the rest next year. Good luck!