MLA
Curtis, William H. Letter from William H. Curtis (Jefferson City, Missouri) to His Father. William H. Curtis Was a Quartermaster's Clerk in the 58th Regiment Illinois Volunteer Infantry - February 18, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 18. https://digital.library.emory.edu/purl/658pc8681q-cor.
APA, 6th edition
Curtis, W. H. (1864). Letter from William H. Curtis (Jefferson City, Missouri) to his father. William H. Curtis was a quartermaster's clerk in the 58th Regiment Illinois Volunteer Infantry - February 18, 1864 (Box 19, Folder 18) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/658pc8681q-cor.
Chicago
Curtis, William H. “Letter from William H. Curtis (Jefferson City, Missouri) to His Father. William H. Curtis Was a Quartermaster's Clerk in the 58th Regiment Illinois Volunteer Infantry - February 18, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 19, Folder 18. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/658pc8681q-cor.