Once you get your parking sensors, they should come with instructions, detailing how far from the ground they should be. I think mine recommended 50-75cm. Too low or too high and it'll pick up too much and continuously beep or won't pick up anything and not beep at all.
I personally put a bit of masking tape where the sensors would go, dropped the back seats and hung out the back with someone else holding the sensors in position while a third person went round checking there were no blind spots before the bumper was drilled. We found that the only blind spot was rediculously close to the bumper, where the tow bar would be. But if there were any, it could be rectified before drilling.