1) Make sure the process has no input objects.
2) The first rule should be
FIND Patient WHERE Patient.BalanceDue>0
3) In the second rule you will probably want to initialize object OverduePaymentLetter with values from a particular Patient, for example:
CREATE OverduePaymentLetter FOR EACH Patient WITH OverduePaymentLetter.Patient = Patient
The User Guide has a good description and examples of implementing similar processes. Please use it.