Literature searching for evidence synthesis is different than doing a general literature search, since it needs to be systematic, and the researcher needs to be very transparent with the search process in order for the research to be reproducible. Bramer, et. al., 2018* provide suggested steps for a systematic literature search:
In addition to these suggestions from Bramer, et. al. the Libraries often recommends that researchers have 'gold standard' articles (articles that are definitely relevant to your search) so you can check that your searches in each database are including all of these articles in your search results (otherwise your search is too narrow). Researchers might also want to consider US/UK spellings, plurals/singulars for search terms, abbreviations, etc.
*Bramer WM, de Jonge GB, Rethlefsen ML, Mast F, Kleijnen J. A systematic approach to searching: an efficient and complete method to develop literature searches. J Med Libr Assoc. 2018 Oct;106(4):531-541. doi: 10.5195/jmla.2018.283. Epub 2018 Oct 1. PMID: 30271302; PMCID: PMC6148622.
It's important to remember that keywords and index terms are not the same things, and you'll want to consider both in your search strategy. You'll also want to remember that index terms are particular to a specific database.
Keywords are often helpful in broadening your search and can be searched for in everything from article titles and abstracts to full text searching.
Index terms (or subject terms) are more likely to focus your search as they are particular terms applied to an article by an indexer.