Force Rollup Calculation with Workflow Tools

I have a Payment entity with a 1:N relationship with Payment Line Item.  The Payment entity has a rollup field totaling all the payment line items.

I want to implement the WorkflowTools solution which forces the rollup, but cannot figure out how to configure it.  Yes, I've read the instructions but they are not clear.

1. Which entity does the workflow go on, the parent or the child?  I tried it on the child and get an error, so assume it goes on the parent. In my scenario, the Payment entity.
2. Is the workflow real-time?
3. What is the trigger for the workflow; create, a field change?

Thanks for the help!