MLA
Barfield, William. Letter from William Barfield (Petersburg, Virginia) to His Sister - December 15, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 19. https://digital.library.emory.edu/purl/692t4b8hvw-cor.
APA, 6th edition
Barfield, W. (1864). Letter from William Barfield (Petersburg, Virginia) to his sister - December 15, 1864 (Box 1, Folder 19) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/692t4b8hvw-cor.
Chicago
Barfield, William. “Letter from William Barfield (Petersburg, Virginia) to His Sister - December 15, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 1, Folder 19. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/692t4b8hvw-cor.