MLA
Eugenia. Letter from Eugenia [Last Name Unknown] (Macon, Georgia), to Her Father - April 22, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 21. https://digital.library.emory.edu/purl/392g79cpq8-cor.
APA, 6th edition
E. (1864). Letter from Eugenia [last name unknown] (Macon, Georgia), to her father - April 22, 1864 (Box 3, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/392g79cpq8-cor.
Chicago
Eugenia. “Letter from Eugenia [Last Name Unknown] (Macon, Georgia), to Her Father - April 22, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 3, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/392g79cpq8-cor.