How to set up additional validation rules in enterprise journals

Summary:

How to set up new validation logic in enterprise journals

Content (please ensure you mask any confidential information):

I am trying to set up enterprise journals as one-stop input point of manual journal entry. This will not connect to ERP system directly though users would like to run some additional validation on EJ before approval/output to interface. Is there anyway to add validation rules on journal templates? I could find external validation flag in target definition but not sure how I should proceed to set up.

It would be appreciated if anybody help me to understand the set up steps and/or input for any other way to add validation rules.