You must set your brake controller per its particular instruction manual, there are too many different kinds to let us give you general instructions for any kind. Personally I have a Prodigy. The Prodigy setup recommendation is to drive slowly (20-25 MPH) on a unpaved road and you operate the brakes manually at full power with a knob/lever on the controller. You set the full power braking just below trailer brake lockup. This generally is way less than 12 volts on my Edge this only takes the controller set to about 5. The voltage will depend on how many electric brakes your trailer have and its weight, Prodigy suggests you start at 6.
Most controllers are like the Prodigy and will never apply that full power unless you panic brake really hard. They sense the rate you are de-accelerating and apply brake voltage based on sensing how hard you are braking. But you want to get that full power setting right so that your trailer brakes don't lock up as once they lock up trailer wheels and tires are sliding and no longer giving the same "help" on stopping that they were before lockup.
The Prodigy also has a boost setting that programs it to "know" the weight of your trailer vs. your tow vehicle and apply the trailer brakes to match. If your trailer brake controller has that feature you need to set it up right, too.
