Click on the Advanced link under the PubMed search box.
In the top search box (called Add terms to the query box), enter one search term and add synonyms/related terms with the word 'or' between.
Then click the Add button.
Also in the top box (called Add terms to the query box), add your next search term, and add its synonyms/related terms with the word 'or' between.
Then click the button AND:
What PubMed will do is, in the second box (called the Query box), it will place your two sets of synonyms together with the word AND between them. It will automatically put parentheses around each set of synonyms so that it looks like this:
Now you can hit the Search button on the right to see your results!
Databases like CINAHL and PsycArticles will offer synonyms for your search! You just need to start typing your first word or phrase and pause. You can then select one of their drop-down options. Feel free to also type any additional terms; just put the word 'or' between each phrase.
Go down to the next search box and type your second term. Again, pause to see what suggestions are listed in the drop-down menu.
Once your search is set up, just hit the green Search button for articles!