When writing a letter to a granddaughter, write from the heart and stay true to yourself. Make sure your writing is legible, but don't worry too much about crafting the perfect letter.
First, choose stationary that is representative of you. This paper should be of fairly heavy stock. Next, consider what you wish to convey in your letter. If it helps to organize your thoughts before putting a pen to paper, type up a draft of your letter in a word-processing application before writing the letter by hand.
Since this letter may be a cherished keepsake for years to come, make sure to include the date at the top of the page. Choose a salutation that reflects your relationship with your grandchild. Write legibly, but in your own handwriting. Stick to print rather than cursive, especially if the child will read your letter without a parent's help.
Begin the letter with a loving statement or a question about the child's life. State why you are writing the letter, and then expand from there. Don't edit yourself too harshly. Instead, write as if you are having a conversation with the child. Close with another loving statement or a special farewell you share, and include your signature.