How to add additional Invoices to an existing PPR?

Hello,

Can anyone give me step-by-step instructions on how to add invoices to an existing PPR? Instructions in Note id: 462791.1 are not clear for this manner.
Before answering this question, please read the business scenario below and provide your answer according to this scenario.

Business Scenario:
1) Enter, validate and approve a few invoices
2) Create a Payment Process Request named PPRTEST1. Make sure payments are created and awaiting for review & approval.
3) Enter, validate and approve other invoices, say INVADD1 and INVADD2
4) How to add invoices created in step 3 to the PPR created in step 2?

Thanks in advance;