How do I cite a letter from a personal collection that is not addressed to me?

Follow the MLA format template. List the author of the letter in the “Author” slot and provide a description of the letter in the “Title of source” slot. Include the recipient’s name in the description. Then list the date of the letter, if known. In the optional-element slot at the end of the entry, indicate that the letter is in a personal collection rather than an archive:

Grant, Samuel. Letter to Theodore Grant. 10 Oct. 1946. Personal collection of Amy Grant.