MLA
Sickel, Horatio Gates. Letter from Horatio Gates Sickel (Near Bell Plain, Virginia) to Union General George A. McCall. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 20, Folder 23. https://digital.library.emory.edu/purl/979h44j25k-cor.
APA, 6th edition
Sickel, H. G. (1862). Letter from Horatio Gates Sickel (Near Bell Plain, Virginia) to Union general George A. McCall (Box 20, Folder 23) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/979h44j25k-cor.
Chicago
Sickel, Horatio Gates. “Letter from Horatio Gates Sickel (Near Bell Plain, Virginia) to Union General George A. McCall.” Letters (correspondence). Civil War Collection, 1862. Box 20, Folder 23. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/979h44j25k-cor.