The published literature on even narrow topics is very large. In order to cover as much of it as possible in your review you need an efficient search method. Keyword searching is one such method. Break your research question(s) down into individual concepts to create search terms.
Spend some time thinking about the ideas behind your research question. Develop a list of keywords which will generate published articles on your topic. If your topic question is "Is family therapy an effective treatment for anorexia nervosa?" then make a list of the concepts contained in it, eg.,