Letter from the Sell Collection
The letter displayed to the right is from the John M. Sell Civil War Collection. To build an MLA-style citation for this letter, you might fill the practice template out like this:
Full Citation
The citation would look like this in your works cited list:
Sell, John M. Letter to William Sell. John M. Sell Civil War Collection, 3 November 1861. Box 1, Folder 3, Special Collections and Archives, Universtiy Library, California State University, Northridge, Los Angeles.
See the MLA Handbook pages 30-36 for more information.