ummm....what do you want to know? Just press the transmit button and talk like a real pilot!
But seriously, could you be a bit more specific about "how to use Roger Wilco"?
You'll find very good tutorials on the Vatsim site on what you're supposed to be saying (if that's what you want to know). Basically it is very much like what you hear in the game when you "talk" to ATC except now you'll have to do it yourself

You'll love it.
Cheers
AJ