Click on the Advanced link under the PubMed search box and look at the History for the current session.
The search results appear in chronological order (most recent first). Combine results from different searches by clicking on the Actions meatballs menu and "Add query" to the search builder. Add another term by clicking the next search term and "Add with AND". Change to OR or NOT as needed. The terms will be added to the Query box directly above the History section. Click Search.