The UNH Library Finding Articles & Journals page, https://www.library.unh.edu/find/articles-journals, has good tips.
Please note that the DOI search in the Citation Linker ("Fetch It") is not currently functional. You can search a DOI in the regular Search Box; if it works, great! If not, a good way is to go to crossref.org, and do a Metadata search on the DOI -- you can put other key article metadata into the library search or citation linker to see if we have the article linked. If not, just create an InterLibrary Loan request with the metadata.