Fishbone diagrams, also known as cause-and-effect diagrams, are about organizing possible causes behind a given problem. Of course, all possibilities will need to be proved or disproved during the Analyze phase of the project.
See the fishbone diagram excel template page for a template with instructions, and our recently added training Powerpoint that you can use for both learning and training others.
If the team is struggling with possible causes to list in a fishbone diagram, consider starting with some major problem areas and completing a 5-Why problem solving exercise for each one.