If you are using Eclipse with the m2eclipse plugin, simply launch your build with Debug As… instead of Run as….
If you are not using m2eclipse, run mvnDebug instead of mvn (for Maven 2.0.8+) and attach a remote debugger on port 8000. For Maven 2.0.8<, add the remote debuggin options to the start script.
Of course, you need to import the sources of the plugin in your workspace.
See also
- Developing and debugging Maven plugins
- Dealing with Eclipse-based IDE