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 - September 16, 1866. 1866. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 55. https://digital.library.emory.edu/purl/9354f4qs51-cor.
APA, 6th edition
Tillinghast, E. L. (1866). Letter from Edwin L. Tillinghast to his sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Edwin served in the 3rd South Carolina Cavalry - September 16, 1866 (Box 6, Folder 55) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/9354f4qs51-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 - September 16, 1866.” Letters (correspondence). Civil War Collection, 1866. Box 6, Folder 55. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/9354f4qs51-cor.