The MLA Handbook identifies nine core elements for entries in a works cited list. Some of these elements will be infrequently used for citing archival collections, while others will almost always be used. As you build your citations, keep the following in mind:
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, Oviatt Library, California State University, Northridge, Los Angeles.
See the MLA Handbook pages 30-36 for more information.