Share her your memories of when she visited last time and say you miss her and that you're looking forward to seeing her again soon. Or, perhaps there is some other thing she does that you'd like to praise. There are some things to bear in mind when writing the letter, such as:  Don't be too sentimental while writing; this spoils the natural effect of your message. Don't give bad expressions about anyone whom she loves. Avoid using slang or curse words. Include a promise from you to do something unexpected for her (something she likes).
Start with dear beloved granny/grandmother. Write about your favorite dishes that she prepares if you like food. Take care with your writing. End on a positive note.