MLA
Lanier, J. S. Letter from J. S. Lanier (Camp near Cedar Creek) to "Miss Linie" - April 20, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 35. https://digital.library.emory.edu/purl/477sn02w57-cor.
APA, 6th edition
Lanier, J. S. (1864). Letter from J. S. Lanier (camp near Cedar Creek) to "Miss Linie" - April 20, 1864 (Box 4, Folder 35) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/477sn02w57-cor.
Chicago
Lanier, J. S. “Letter from J. S. Lanier (Camp near Cedar Creek) to ‘Miss Linie’ - April 20, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 4, Folder 35. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/477sn02w57-cor.