Try preliminary searches. Keep in mind that "Subjects" are not keywords and not the same as the subject headings used in the library. You may need to be creative in your search.
Identify the location. If you search the archives database "Archon," and find record series that look promising, make note of the location of the files.
Digital records. When materials are digitized by our archives, the title of those documents may begin with the words "Digital surrogate of..."
When all else fails - ask an archivist!!
University of Illinois
The Rare Book & Manuscript Library has a number of primary source material relating to World War I. A subject heading worth browsing in the Classic Catalog is "World War, 1914-1918 Personal narratives, American." Make sure to limit the search to Rare Book & Manuscript Library. Note: the Rare Books & Manuscript Library is only available in the Classic Catalog.
French World War I Posters at the Main Library contains digitized versions of 105 posters published in France during the First World War.
The Champaign County Historical Archives has a new exhibit!
Nettie Mills Papers at the Champaign County Historical Archives contains letters, military service records, and reminiscences. Nettie Dorothy Irle was an American nurse during her service with the Army Nursing Corps (ANC)/American Expeditionary Force in World War I.
Miller, D. (2014). "Illinois Waiting Till Her Prairie Boys Come Home": The Poetry of World War I Soldier Audie V. Wilborn. Champaign County (IL) Genealogical Society Quarterly 35.4 (March 2014): 123-126.
British Army War Diaries at the UK National Archives are war diaries of the first three cavalry and the first seven infantry divisions of the British Army in the First World War.
World War I Document Archive at Brigham Young University, assembled by volunteers of the World War I Military History List (WWI-L), has an international focus.
First World War multimedia history devotes a section of their website to primary source material--many from government sources--that documents the course of the war.
A Guide to WWI Materials at the Library of Congress provides digital collections as well as links to external Web sites focusing on World War I and a bibliography containing selections for both general and younger readers.