MLA
Wylly, Alexander W. Letter from Alexander W. Wylly (near Milltown, Virginia[?]), to "Very Dear Wife and Beloved Children," Containing His Last Wishes (Although He Did Not Die in the War) - June 22, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 7 Folder 46. https://digital.library.emory.edu/purl/194hmgqpmx-cor.
APA, 6th edition
Wylly, A. W. (1864). Letter from Alexander W. Wylly (near Milltown, Virginia[?]), to "very dear wife and beloved children," containing his last wishes (although he did not die in the war) - June 22, 1864 (Box 7 Folder 46) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/194hmgqpmx-cor.
Chicago
Wylly, Alexander W. “Letter from Alexander W. Wylly (near Milltown, Virginia[?]), to ‘Very Dear Wife and Beloved Children," Containing His Last Wishes (Although He Did Not Die in the War) - June 22, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 7 Folder 46. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/194hmgqpmx-cor.