MLA
Autt, Thomas H. Letter from Thomas H. Autt (Montevallo, Alabama) to His Sister. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 13. https://digital.library.emory.edu/purl/969866t2k3-cor.
APA, 6th edition
Autt, T. H. (1864). Letter from Thomas H. Autt (Montevallo, Alabama) to his sister (Box 1, Folder 13) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/969866t2k3-cor.
Chicago
Autt, Thomas H. “Letter from Thomas H. Autt (Montevallo, Alabama) to His Sister.” Letters (correspondence). Civil War Collection, 1864. Box 1, Folder 13. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/969866t2k3-cor.