MLA
Curtis, George C. Letter from George C. Curtis (Harewood Hospital, Washington, D. C.) to His Father Regarding His Location. Curtis' Regiment, the Identify of Which Is Not given, Was with the Army of the Potomac near Fredericksburg in 1863 - April 23, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 17. https://digital.library.emory.edu/purl/081wdbrw51-cor.
APA, 6th edition
Curtis, G. C. (1863). Letter from George C. Curtis (Harewood Hospital, Washington, D. C.) to his father regarding his location. Curtis' regiment, the identify of which is not given, was with the Army of the Potomac near Fredericksburg in 1863 - April 23, 1863 (Box 19, Folder 17) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/081wdbrw51-cor.
Chicago
Curtis, George C. “Letter from George C. Curtis (Harewood Hospital, Washington, D. C.) to His Father Regarding His Location. Curtis' Regiment, the Identify of Which Is Not given, Was with the Army of the Potomac near Fredericksburg in 1863 - April 23, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 19, Folder 17. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/081wdbrw51-cor.