Properly:
Power cable from the battery or splice into something somewhere. Relay to turn the unit on, use power from the reverse light.
Cut the wholes in the bumpers and fit the sensors, wire them up and turn it on.
A lot of new cars have marks or crosses on the back of the bumper where to drill so the job is easy peasy.
Edit: its a sprinter van, i was working on a 58plate one a few weeks ago and had to fit sensors and im pretty sure the guy i was working with said there is marks on the back on where to drill