Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Grandma/Grandpa/your choice of title],
Happy birthday to one of my favorite people in the entire world! I hope that you have as much fun on your birthday as I always have when I am with you.
Thank you for all the things you do for me throughout the year - for the phone calls and cards, for the special time you manage to spend with me. Thank you for listening to me and always making me feel special and loved.
Here's to a wonderful birthday, and for many more years to come that I am able to spend with you in my life.