It's pretty easy when you understand the fundamentals. Basically you want controllers mapped to actions that modify data, whether that data's in a database, XML file, CSV file, text file depends on your models.
I created a simple API framework starter that you can take a look at and get started with: https://github.com/martinbean/api-framework
I recently used FRAPI to do this, http://getfrapi.com/
It's focus is on burying all the donkey work and letting you concentrate on what the API really needs to DO! ... and as an added plus, it generates an API specification with no extra effort required.