How to start invoice approval based on contracts

How do I create workflow approvals for invoices using the contract administrator?

Workflow

New to Functionality

1. Create a contract with a contract administrator and workflow set to approve
2. Create a Workflow Business Rule for 
   1. Process -API
   2. Approver-the administrator on contract
   3. Type - APP - Approver and amount

With resolution above, any invoices keyed in against that contract will go through the administrator workflow. If invoices are created not against that contract, that rule created will not trigger.