MLA
Ammel, Daniel. Letter from Daniel Ammell (Fort Royal, South Carolina) to Dr. Arnold - January 28, 1865. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 1. https://digital.library.emory.edu/purl/10966t1h69-cor.
APA, 6th edition
Ammel, D. (1865). Letter from Daniel Ammell (Fort Royal, South Carolina) to Dr. Arnold - January 28, 1865 (Box 19, Folder 1) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/10966t1h69-cor.
Chicago
Ammel, Daniel. “Letter from Daniel Ammell (Fort Royal, South Carolina) to Dr. Arnold - January 28, 1865.” Letters (correspondence). Civil War Collection, 1865. Box 19, Folder 1. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/10966t1h69-cor.