I would suppose you select heavy electric, but only you know if you have electric or electric over hydraulic. Then, at 20-25 mph, fully apply only trailer brake. If it locks up turn the gain down and do it again. If it doesn't lock up adjust the gain up and do it again. You're trying to set the gain such that if you cram the brakes on in an emergency the trailer brakes will lock up with the truck brakes, but only just. It really is a trial and error sort of adjustment.
When you get it set test it again a time or two. At higher speeds when you need to manually hit the trailer brakes to stop a sway or control a descent it will have a lot of brake available but not quite enough to skid.
