MLA
Campbell, Charles G. Letter from Charles G. Campbell (Headquarters Port St. Marks, Florida) to Captain Pope. Campbell Formed the Independent Company of Georgia Siege Artillery in Bainbridge, Georgia and St. Marks, Florida. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 4. https://digital.library.emory.edu/purl/279v15dw44-cor.
APA, 6th edition
Campbell, C. G. (1863). Letter from Charles G. Campbell (Headquarters Port St. Marks, Florida) to Captain Pope. Campbell formed the Independent Company of Georgia Siege Artillery in Bainbridge, Georgia and St. Marks, Florida (Box 2, Folder 4) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/279v15dw44-cor.
Chicago
Campbell, Charles G. “Letter from Charles G. Campbell (Headquarters Port St. Marks, Florida) to Captain Pope. Campbell Formed the Independent Company of Georgia Siege Artillery in Bainbridge, Georgia and St. Marks, Florida.” Letters (correspondence). Civil War Collection, 1863. Box 2, Folder 4. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/279v15dw44-cor.