Once you have knowledge base yo should create session from it and then start your process by id:
StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
ksession.startProcess("process.id");
// or start with params
ksession.startProcess("process.id", params);
If you need persistence than you should use JPAKnowledgeService to create session.
StatefulKnowledgeSession ksession = JPAKnowledgeService.newStatefulKnowledgeSession( kbase, null, env );
For details take a look at java docs and documentation
HTH