MLA
Arnsdorff, Gideon A. Letter from Gidieon A. Arnsdorff to "Dear Mother" - May 18, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 10. https://digital.library.emory.edu/purl/003cz8wbhw-cor.
APA, 6th edition
Arnsdorff, G. A. (1863). Letter from Gidieon A. Arnsdorff to "Dear Mother" - May 18, 1863 (Box 1, Folder 10) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/003cz8wbhw-cor.
Chicago
Arnsdorff, Gideon A. “Letter from Gidieon A. Arnsdorff to ‘Dear Mother’ - May 18, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 1, Folder 10. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/003cz8wbhw-cor.