MLA
Johnston, O. H. Letter from O. H. Johnston (Camp Clark, Shieldsboro to "My Dear Wife" - September 6, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 19. https://digital.library.emory.edu/purl/409j3txb6w-cor.
APA, 6th edition
Johnston, O. H. (1861). Letter from O. H. Johnston (Camp Clark, Shieldsboro to "My Dear Wife" - September 6, 1861 (Box 4, Folder 19) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/409j3txb6w-cor.
Chicago
Johnston, O. H. “Letter from O. H. Johnston (Camp Clark, Shieldsboro to ‘My Dear Wife’ - September 6, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 4, Folder 19. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/409j3txb6w-cor.