The essence of talking to LCDd is quite simple. First you will need to connect to the LCDproc port (usually 13666) on the correct IP address (by default localhost). Once you have established the connection you should say "hello", to let LCDd know you are a good guy. It will respond by telling some LCDproc data, like version and screen width and height. Now your session is open and you can start sending 'real' commands.
LCDd can send a number of strings itself. As a response to your commands, it will usually send a "success" string, or a string starting with "huh" in case of any error. See further below for other strings sent by LCDd.
You can test all these commands by opening a TCP/IP connection manually, like with:
telnet localhost 13666
This way, you can check how the various commands work. It's in this case best to have no other clients. If you do have other clients, you will receive "listen" and "ignore" messages that will disturb your typing.