This is also a critical step. Be ready to do multiple searches with different combinations of terms -- don't stop after just one try.
If you get too many hits, try narrowing your search by adding additional terms or applying limits.
If you get too few or no hits, broaden your search by using fewer or more general terms.
Browse your results to determine how useful they seem to be. Look for additional terms or synonyms you may not have thought of and search them too.
If your results truly are very large even after you've focused it well, try limiting to Reviews, which will help you get an overview and identify the best articles to read. Both SciFinder and Web of Science have Review filter options that make this an easy step.