I guess you have to:
- Create work item definition with some name, for example "MyTask".
- Add a jar file with your WorkItemHandler implementation.
- Create a session template attaching your custom work item handler to "My task".
https://github.com/droolsjbpm/jbpm/blob/5.4.x/jbpm-gwt/jbpm-gwt-core/src/test/resources/default.session.template
This will make that the Knowledge Session is able to execute the My Task node.
But I think, to write your own work item handlers, you should have to use eclipse or some other IDE.
HTH
Demian