This is a compile of a few links you will find on the blog is you are interested in learning some of the basics of setting up and using External catalogs aka Punch-Out catalogs in procurement.
Here are a couple of posts on the experience of using an external catalog.
Purchase Requisition with External Catalog AKA Punch Out.
External Item Handling with Punch Out Catalog
Setup External Catalog
Here are a couple of posts on setting up an external catalog
External Catalog AKA Punch Out Setup
Procurement External Catalog AKA Punch Out Concept and Setup
Punch Out Catalog Message Handler Protocol Setup
Return URL Setup on Punch Out Catalog
Modifying a Message
This posts covers an example of things that you might need to consider if the message between the supplier site needs to be modified.