Simply sit down and write her a letter. Tell her in the letter why you want to honor her by listing some of the good things she has done for you, important lessons she has taught you, special memories that have shown her love for you.
Be specific and let her know why you are grateful to her for giving you life, raising you and supporting you. I guarantee that any Mom on the planet would love to receive this and will treasure you taking the time to let her know how you feel.