Adding FedEx shipping options to existing checkout - where to start?

I am trying to include FedEx shipping and real time rate lookup options into my existing website but having trouble even finding out where to begin.

I found several posts about how to implement snippets and code etc but they seem to be starting somehow in the middle of the process.

We have a working checkout with price based shipping rules and I like to add the real time lookup and selection of FedEx expedited services. Can I use my existing eCart object or do I have to remove this and start with a new eCart object?

I figured that I have to start with Kate Ford’s online tutorial about online shipping solutions and obtain a FedEx developer test key and downloaded the rate service code from the FedEx developer site but already have trouble with the part “past the developer test key, etc into the sample code”.

So I am kind of lost and don’t even know whether this is the right way to start. Is there any tutorial or guideline that explains the steps of adding FedEx shipping options into an existing checkout process?

Thanks so much.

Joerg