You could start the letter by expressing your love and appreciation for your mom.
You could share some of your favorite memories with her and how she has impacted your life.
You could also thank her for all the sacrifices she has made for you and for always being there to support you. It might be nice to include some specific examples of things she has done for you that you're grateful for.
You could end the letter by telling her how much she means to you and how much you love her.