MLA
Curtis, William H. Letter from William H. Curtis (Camp near New Orleans, Louisiana) to His Father and Mother. William H. Curtis Was a Quartermaster's Clerk in the 58th Regiment Illinois Volunteer Infantry - March 6, 1865. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 18. https://digital.library.emory.edu/purl/535t76hfx9-cor.
APA, 6th edition
Curtis, W. H. (1865). Letter from William H. Curtis (Camp near New Orleans, Louisiana) to his father and mother. William H. Curtis was a quartermaster's clerk in the 58th Regiment Illinois Volunteer Infantry - March 6, 1865 (Box 19, Folder 18) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/535t76hfx9-cor.
Chicago
Curtis, William H. “Letter from William H. Curtis (Camp near New Orleans, Louisiana) to His Father and Mother. William H. Curtis Was a Quartermaster's Clerk in the 58th Regiment Illinois Volunteer Infantry - March 6, 1865.” Letters (correspondence). Civil War Collection, 1865. Box 19, Folder 18. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/535t76hfx9-cor.