Hi,
the best and easiest way is to use ant script that is delivered with jBPM distribution.
Please take a look at User guide for more details:http://docs.jboss.com/jbpm/v4/userguide/html_single/#jboss
jBPM has several data base setups and one of them is Postgre. So you need to provide what data base you would like to setup while running ant (-Ddatabase=postgresql)
HTH
Maciej