MLA
Tillinghast, Edwin L. Letter from Edwin L. Tillinghast to His Sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Edwin Served in the 3rd South Carolina Cavalry - August 5, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 55. https://digital.library.emory.edu/purl/849t1g1kk2-cor.
APA, 6th edition
Tillinghast, E. L. (1861). Letter from Edwin L. Tillinghast to his sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Edwin served in the 3rd South Carolina Cavalry - August 5, 1861 (Box 6, Folder 55) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/849t1g1kk2-cor.
Chicago
Tillinghast, Edwin L. “Letter from Edwin L. Tillinghast to His Sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Edwin Served in the 3rd South Carolina Cavalry - August 5, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 6, Folder 55. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/849t1g1kk2-cor.