написать открытку по английскому Dear Grandmq

Речь идет о бабушке? Тогда grandma или grandmother.

Dear Grandmother,
I am writing you a letter. We have not seen each other for a long time. How are you? I am fine. I'm walking, playing with my friends. Our family misses you. When do you come? We will wait for you very much.

With best wishes, your family and friends.