If you found your artifact online (or found the digital copy of the artifact you looked at in person), a lot of the information you need for your citation is listed on the artifact's webpage.
Let's say you want to cite the flyer for Jean McGuire's candidacy for the Boston School Committee and you need to find the box and folder number in which it resides, as well as the collection number. On the webpage for the artifact, scroll down until you find the identifier, which is highlighted below.

We can break down the identifier to find the collection number, the box number, and the folder number.

Once we pull all the pieces together, the full citation for this flyer is:
Jean McGuire for Boston School Committee flyer.
Metropolitan Council for Educational Opportunity, Inc. records. (M101)
University Libraries
Archives and Special Collections
Northeastern University, Boston, Massachusetts
Box 42, folder 5.
If you have questions about citing your artifact, please feel free to contact us and we would be happy to help you! You can call us at (617) 373-2351 or fill out the web form.