If you would like to wish your grandma on her birthday, here is a simple template you can use to write a birthday letter to grandma.
Dearest [name],
I cherish every moment I spend with you. Thus, I wish you have an awesome birthday. If you want to something like go to the park or eat at your favorite restaurant then I will gladly make arrangements. I would love to be with you on your birthday.
Nothing makes me happier than seeing you happy. You took care of me when I was young so I will see to it to return the favor every chance I get. You are also my best friend and you are the first person I go to when I have problems.
Cheers to a spectacular birthday. Once this day ends, I look forward to more birthdays in the coming years.
Sincerely yours,
[name]