MLA
Clark, Walter H. Letter from Walter H. Clark to William H. Clark - August 28, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 21. https://digital.library.emory.edu/purl/583kwh71r9-cor.
APA, 6th edition
Clark, W. H. (1862). Letter from Walter H. Clark to William H. Clark - August 28, 1862 (Box 2, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/583kwh71r9-cor.
Chicago
Clark, Walter H. “Letter from Walter H. Clark to William H. Clark - August 28, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 2, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/583kwh71r9-cor.