MLA
Armistead, John. Letter from John Armistead to His Sister-in-Law Fannie[?] (Richmond, Virginia) - June 4, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 25. https://digital.library.emory.edu/purl/5805x69q9f-cor.
APA, 6th edition
Armistead, J. (1861). Letter from John Armistead to his sister-in-law Fannie[?] (Richmond, Virginia) - June 4, 1861 (Box 4, Folder 25) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/5805x69q9f-cor.
Chicago
Armistead, John. “Letter from John Armistead to His Sister-in-Law Fannie[?] (Richmond, Virginia) - June 4, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 4, Folder 25. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/5805x69q9f-cor.