One approach is to take one of the self-help books, such as my The Forgiving Life book published by the American Psychological Association. I recommend that you read it first. If you think it is appropriate for your mother, then share it with her and point out some of the sections in the book that proved helpful to you. Your mother might get interested and, if so, this would give her a chance to work through the forgiveness process.
Learn more at The Forgiving Life.