MLA
McHollaway, Robert. Letter from Robert McHollaway to "Dear Brother" - December 11, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 5, Folder 5. https://digital.library.emory.edu/purl/716pzgmtdd-cor.
APA, 6th edition
McHollaway, R. (1862). Letter from Robert McHollaway to "Dear brother" - December 11, 1862 (Box 5, Folder 5) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/716pzgmtdd-cor.
Chicago
McHollaway, Robert. “Letter from Robert McHollaway to ‘Dear Brother’ - December 11, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 5, Folder 5. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/716pzgmtdd-cor.