When doing mine, I use a jumper with two alligator clips.
I have one constant power wire running to the rear of my truck to power my break-away battery. I find the constant on my truck side and clip the power wire to it. Then I clip it to a bullet connector in-line with my blue break wire. Basically I disconnect it and clip the alligator clip to it.
I've got the trailer jacked up and on stands and I check each wheel to see if the magnet has engaged and the tire isn't moving.