MLA
Smart, Rees P. Letter from Rees P. Smart (Headquarters, Walker's Tent, Metairie Race Course, New Orleans, Louisiana) to His Sister, Miss Jane Smart. The Smart Family Lived in Northern Louisiana - May 8, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 26. https://digital.library.emory.edu/purl/3009w0vtss-cor.
APA, 6th edition
Smart, R. P. (1861). Letter from Rees P. Smart (Headquarters, Walker's Tent, Metairie Race Course, New Orleans, Louisiana) to his sister, Miss Jane Smart. The Smart family lived in northern Louisiana - May 8, 1861 (Box 6, Folder 26) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3009w0vtss-cor.
Chicago
Smart, Rees P. “Letter from Rees P. Smart (Headquarters, Walker's Tent, Metairie Race Course, New Orleans, Louisiana) to His Sister, Miss Jane Smart. The Smart Family Lived in Northern Louisiana - May 8, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 6, Folder 26. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3009w0vtss-cor.