MLA
Sandlin, Alfred W. Letter from Alfred W. Sandlin to His Wife, Lucinda E. Sandlin (Mooreville, Mississippi). Alfred W. Sandlin Served in the 1st Mississippi Infantry Regiment and Was Captured at Port Hudson, Louisiana on July 7, 1863 - March 17, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 11. https://digital.library.emory.edu/purl/764gmsbd32-cor.
APA, 6th edition
Sandlin, A. W. (1863). Letter from Alfred W. Sandlin to his wife, Lucinda E. Sandlin (Mooreville, Mississippi). Alfred W. Sandlin served in the 1st Mississippi Infantry Regiment and was captured at Port Hudson, Louisiana on July 7, 1863 - March 17, 1863 (Box 6, Folder 11) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/764gmsbd32-cor.
Chicago
Sandlin, Alfred W. “Letter from Alfred W. Sandlin to His Wife, Lucinda E. Sandlin (Mooreville, Mississippi). Alfred W. Sandlin Served in the 1st Mississippi Infantry Regiment and Was Captured at Port Hudson, Louisiana on July 7, 1863 - March 17, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 6, Folder 11. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/764gmsbd32-cor.