MLA
Gwillin, Mattie[?]. Letter from Mattie Gwillin[?] (Oak Hill near Decatur, Georgia) to "Dear Cousin Jimmie" - July 17, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 52. https://digital.library.emory.edu/purl/846d2548h6-cor.
APA, 6th edition
Gwillin, M. (1862). Letter from Mattie Gwillin[?] (Oak Hill near Decatur, Georgia) to "Dear Cousin Jimmie" - July 17, 1862 (Box 2, Folder 52) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/846d2548h6-cor.
Chicago
Gwillin, Mattie[?]. “Letter from Mattie Gwillin[?] (Oak Hill near Decatur, Georgia) to ‘Dear Cousin Jimmie’ - July 17, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 2, Folder 52. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/846d2548h6-cor.