Ritchie provides general compensation instructions within the instruction packets that come with most compass series.
Most Ritchie compasses have internal comp rods that can correct for +/- 15° of deviation. If your compass has a considerable amount of deviation (+15°), the built-in compensators will not be enough. Hiring a professional compass adjuster may be the way to go.
A professional compass adjuster is trained to help you locate potential causes of deviation and make recommendations to reduce them. Your local marina or harbor master should be able to put you in touch with a compass adjuster.