MLA
Armistead, Mary . Letter from Mary Armistead to Her Sister, Fannie Keen (Richmond, Virginia) - July 16, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 25. https://digital.library.emory.edu/purl/668x95x7cm-cor.
APA, 6th edition
Armistead, M. (1863). Letter from Mary Armistead to her sister, Fannie Keen (Richmond, Virginia) - July 16, 1863 (Box 4, Folder 25) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/668x95x7cm-cor.
Chicago
Armistead, Mary . “Letter from Mary Armistead to Her Sister, Fannie Keen (Richmond, Virginia) - July 16, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 4, Folder 25. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/668x95x7cm-cor.