MLA
Howard, Monroe. Letter from Monroe Howard (Fairfax County, Virginia) to His Brother, Dr. Thomas Henry Howard - February 11, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 64. https://digital.library.emory.edu/purl/7032280hbj-cor.
APA, 6th edition
Howard, M. (1862). Letter from Monroe Howard (Fairfax County, Virginia) to his brother, Dr. Thomas Henry Howard - February 11, 1862 (Box 3, Folder 64) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hbj-cor.
Chicago
Howard, Monroe. “Letter from Monroe Howard (Fairfax County, Virginia) to His Brother, Dr. Thomas Henry Howard - February 11, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 3, Folder 64. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hbj-cor.