It's really very simple, I use the 4th channel for an ignition kill switch on my 5th scale. If you are running a receiver battery you're going to have to use a Y cable, the servo in one end and the battery in the other.

Chose which button or switch you want to use for the steering and assign it to channel 4. You'll probably have to play around with the settings and steps to get it to where you want it to be...