You can created them in eclipse or any text/html editor really. There is no specific editor. Designer generates forms for you which is nice if you do not want to do it yourself, or don't want to deal with maintenance of existing forms after updating your process (just re-generated them in Designer after process updates).
As far as where these forms are, look here: https://github.com/droolsjbpm/jbpm/tree/master/jbpm-gwt/jbpm-gwt-form/src/main/resources.