MLA
James [unknown last name]. Letter from James [unknown last name] (Camp Leon, Florida) to his sister, "Dear Sallie" - December 26, 1863. December 26, 1863. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/255z612kp0-cor.
APA, 6th edition
James [unknown last name]. (December 26, 1863)[Letter from James [unknown last name] (Camp Leon, Florida) to his sister, "Dear Sallie" - December 26, 1863]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/255z612kp0-cor.
Chicago
James [unknown last name], Letter from James [unknown last name] (Camp Leon, Florida) to his sister, "Dear Sallie" - December 26, 1863, December 26, 1863, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/255z612kp0-cor.