MLA
Richards, W. L. G. Letter from W. L. G. Richards to "My Dear Sister" - September 26, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 5, Folder 62. https://digital.library.emory.edu/purl/887brv16g9-cor.
APA, 6th edition
Richards, W. L. G. (1861). Letter from W. L. G. Richards to "My Dear Sister" - September 26, 1861 (Box 5, Folder 62) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/887brv16g9-cor.
Chicago
Richards, W. L. G. “Letter from W. L. G. Richards to ‘My Dear Sister’ - September 26, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 5, Folder 62. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/887brv16g9-cor.