MLA
Lanneau, Fleet. Letter from Fleet Lanneau (Charleston, South Carolina) to His Mother. Fleet Lanneau Died Two Weeks after Writing the Letter - 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 36. https://digital.library.emory.edu/purl/071ngf1wm9-cor.
APA, 6th edition
Lanneau, F. (1861). Letter from Fleet Lanneau (Charleston, South Carolina) to his mother. Fleet Lanneau died two weeks after writing the letter - 1861 (Box 4, Folder 36) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/071ngf1wm9-cor.
Chicago
Lanneau, Fleet. “Letter from Fleet Lanneau (Charleston, South Carolina) to His Mother. Fleet Lanneau Died Two Weeks after Writing the Letter - 1861.” Letters (correspondence). Civil War Collection, 1861. Box 4, Folder 36. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/071ngf1wm9-cor.