When using Eclipse, you would need to create the forms yourself. Reference the docs on the naming convention and structure of the forms (http://docs.jboss.org/jbpm/v5.1/userguide/ch11.html#d0e3187) and you can also see some examples of existing forms and processes here: https://github.com/droolsjbpm/jbpm/tree/master/jbpm-examples/src/main/resources/humantask.
The web-based Designer allows you to pre-generate fully functional process and task forms, for more info see this blog post and video: http://blog.athico.com/2011/07/develop-complex-jbpm-processes-all-in.html
If you want Designer to generate your forms, you can upload your existing processes into guvnor and generate the forms, or use Designer to create your new processes.
Hope this helps.