MLA
Cross, Ella. Letter from Ella Cross (Warrior, Alabama) to Her Mother, Brothers and Sisters, Discussing Family Matters and the Costs of Various Products - December 1, 1891. 1891. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 5, Folder 14. https://digital.library.emory.edu/purl/13331zcsm0-cor.
APA, 6th edition
Cross, E. (1891). Letter from Ella Cross (Warrior, Alabama) to her mother, brothers and sisters, discussing family matters and the costs of various products - December 1, 1891 (Box 5, Folder 14) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/13331zcsm0-cor.
Chicago
Cross, Ella. “Letter from Ella Cross (Warrior, Alabama) to Her Mother, Brothers and Sisters, Discussing Family Matters and the Costs of Various Products - December 1, 1891.” Letters (correspondence). Civil War Collection, 1891. Box 5, Folder 14. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/13331zcsm0-cor.