Example:
Articulate the subject you want to investigate:
Therapy for ptsd for people who experience/engage in spouse abuse
Identify the words/phrases that constitute the main topics:
Therapy for ptsd for people who experience/engage in spouse abuse
Start searching using your own vocabulary for your topics
Review some of the full records on the result list and look for PsycINFO subject terms that are synonymous with your search terms
Redo your search combining all the database subject terms you found:
therapy or psychotherapy or treatment or counseling or intervention
AND
"posttraumatic stress disorder"
AND
"partner abuse" or "domestic violence" or "intimate partner violence"