¿Somebody knows how the make the purchasing process all automatical?

We have a list of item that have a price and supplier defined by one year.

When we create a Requisition with these items, the Requisition is approved by the requisitor and the buyer create the Purchase Order and sent it to approve in the hierarchy.

I need that the documents created with this items will follow the next flow automatically:

1.- The requisitor creates a Requisition

2.- The Requisition automatically change to "Approved"

3.- The Requisition automatically creates a Purchase Order and automatically sent the PO to the approbation flow.

4.- The PO will run by the approve hierarchy  flow until its approbation.