Well currently I am mixing the third channel on my Hitec CRX with the throttle channel, this way the brake servo only moves in one direction. This is probably the best way to set it up. However, on my original setup I just used a y cable out of the throttle channel on the rx. Turn the brakes off on your esc and you can use the endpoint on your radio to adjust max throw on the brake servo (just as you would a nitro) because that setting won't effect the esc. Be sure the endpoint is maxxed out for the throttle, or positve endpoint, you only want to mess with the brake, or negative endpoint.
I hope I explained that well enough, if you have any more questions, please feel free to ask, and welcome to the forums