MLA
Shuler, Spartan McCain. Letter from Spartan McCain Shuler (Yorktown, Virginia) to "My Dearest Father". Spartan M. Shuler Served in Company G of the 6th South Carolina Infantry Regiment - April 22, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 21. https://digital.library.emory.edu/purl/211kh189sp-cor.
APA, 6th edition
Shuler, S. M. C. (1862). Letter from Spartan McCain Shuler (Yorktown, Virginia) to "My Dearest Father". Spartan M. Shuler served in Company G of the 6th South Carolina Infantry Regiment - April 22, 1862 (Box 6, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/211kh189sp-cor.
Chicago
Shuler, Spartan McCain. “Letter from Spartan McCain Shuler (Yorktown, Virginia) to ‘My Dearest Father’. Spartan M. Shuler Served in Company G of the 6th South Carolina Infantry Regiment - April 22, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 6, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/211kh189sp-cor.