Go to the search page of Reaxys and perform a search. For an article, under the source tab there will be a link saying view citing articles. Click on the link and it will take you to a page that contains the articles that cite the one that is wanted.
Cited reference searching starts with a specific article, book, or other publication. Usually your first step is to find that publication in the database you are using. That may mean searching the title, but sometimes you'll use the author's name. Since citations for this publication can vary, expect the need to select from among variations or to perform multiple searches and combining the results.