Topic: How do I POST and GET XML to a RESTful service?

I haven't seen a good, clear illustration of step-by-step how to POST and GET (and include Authentication headers) for interacting with an external RESTful service via XML.

I have read about ActiveResource and a ruby-client called rest-client.  But here is what I want to do:

I am trying to POST to a RESTful API (the details are below) XML and an Authentication header based on base64 encode.

I have no idea where to begin.  sad

Here is the basic docs of what the service is I am trying to POST and GET from:

I've started to play around with ActiveResource (does that go in the Models folder)?  But still not sure how to format what I need to POST, where to create the Authentication header, etc.

Initial guidelines would be great.