Refer to the style handbook for the citation format you will be using (MLA, Chicago, APA).
Then follow the format of the item itself: letter, image, pamphlet, book, government document, etc.
The Library of Congress gives examples of how to cite different types of primary sources in these two styles:
If the items is PUBLISHED, refer to the chosen style (MLA, APA, Chicago) citation practices most relevant to the particular genre of your materials, and follow this.
If the material is UNPUBLISHED you should refer to the chosen style as above, and also include: box numbers, folders, collections, archives name, institutional affiliation, location and date accessed.
The following example is based on using MLA Style and Purdue Owl as a guideline:
Genre-appropriate MLA Citation. Box number, Folder number. Unique identifier and collection name. Archives name, Institutional affiliation, Location. Date accessed.
Summers, Clara. Letter to Steven Summers. 29 June 1942. Box 1, Folder 1. MSP 94 Steven and Clara Summers papers. Virginia Kelly Karnes. Archives and Special Collections Research Center, Purdue University Libraries, West Lafayette, IN. 20 May 2013.