How to create a REST based webservice in PI?

Hi Expert,

   I want to expose my PI interface as REST based web service. I also want to use JSON.

   i have already searched the internet but did not get any step by step how to doc or example.

   My interface is simple. It contains userid as input.

   Web service (REST) -> Pi -> Proxy (SAP R/3)

   The user passes the userid as the input to the web service. The PI then calls ABAP proxy in a SAP R/3 system.

   The R/3 returns user details(First name, last name, Age, Address) back.

   How my schema should look like in PI interface? What URL I need to send it to calling application (system that calls the web service)?

   What setups I need to do in REST Sender communication channel in order to implement JSON?

Thanks

Gopal