MLA
Tillinghast, Robert G. Letter from Robert G. Tillinghast (Camp Morgan, Grahamsville, South Carolina) to His Sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Robert Served in the 3rd South Carolina Cavalry - March 13, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 55. https://digital.library.emory.edu/purl/423612jmxm-cor.
APA, 6th edition
Tillinghast, R. G. (1863). Letter from Robert G. Tillinghast (Camp Morgan, Grahamsville, South Carolina) to his sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Robert served in the 3rd South Carolina Cavalry - March 13, 1863 (Box 6, Folder 55) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/423612jmxm-cor.
Chicago
Tillinghast, Robert G. “Letter from Robert G. Tillinghast (Camp Morgan, Grahamsville, South Carolina) to His Sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). Robert Served in the 3rd South Carolina Cavalry - March 13, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 6, Folder 55. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/423612jmxm-cor.