The Mirror mode is a good way to test out your tooling and see the details of your requests without using a proxy.
The API has a simulation mode, it uses hard coded data in responses, but tries to mimic some conditions.
e.g. it expects you to post a specific JSON payload or XML payload and responds 'as if' you sent it. But... it also checks if you sent valid json, or valid xml, and responds based on your headers e.g. returning XML if you ask for it.
The simulator is stateless and does not track your usage, making it deterministic for multiple users.
The simulator is a good place to get started because it will respond nicely... unless you mess up the request syntax.
Automating and Testing a REST API Book
Alan Richardson wrote a book Automating and Testing a REST API
Buying the book helps support this web site and application.
All of the Challenges have solution tutorials that are available on the EvilTester.com blog.
Open Source Workshops
All the material from Alan Richardson's REST API Training workshops have been released to Github.