I've been googling for quite some time now, but I was unable to locate any complete example, with the request & the POST/GET parameters to the Bitcoin JSON-RPC.
How to use the JSON-RPC ? How to use it to make a transaction ?
The list of json RPC commands is located on the wiki: https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_Calls_list
There is a list of json-rpc wrapper libraries on the blockchain wallet API page: https://blockchain.info/api/json_rpc_api
Pick a wrapper library for the environment you develop in and dive in.
To create a transaction, you're going to likely use "sendfrom" or "sendtoaddress".