MLA
Marshall, Michael. Letter from Michael Marshall (Harrison's Landing[?], Virginia) to His Sister, Margaret Marshall. Michael Was a Solider in the United States Army, 17th Infantry Regiment, Company A and Died at the Battle of Globe Tavern in 1864 - July 24, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 23, Folder 20. https://digital.library.emory.edu/purl/815p8czb7v-cor.
APA, 6th edition
Marshall, M. (1862). Letter from Michael Marshall (Harrison's Landing[?], Virginia) to his sister, Margaret Marshall. Michael was a solider in the United States Army, 17th Infantry Regiment, Company A and died at the Battle of Globe Tavern in 1864 - July 24, 1862 (Box 23, Folder 20) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/815p8czb7v-cor.
Chicago
Marshall, Michael. “Letter from Michael Marshall (Harrison's Landing[?], Virginia) to His Sister, Margaret Marshall. Michael Was a Solider in the United States Army, 17th Infantry Regiment, Company A and Died at the Battle of Globe Tavern in 1864 - July 24, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 23, Folder 20. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/815p8czb7v-cor.