ive fitted a few at work,
you should get a drill bit provided with the parking sensor kit you buy to drill the correct size holes into the bumper, if you dont then getting the correct size holes can be a bit of a pain.
best way weve found was going from corner to corner on the bumper and putting masking tape on verticaly, then work out roughly where there going to go and tape the bumper horizontaly too. then measuring between the 2 points youv taped off verticaly and depending on how much sensors you plan on fitting, mark it accordingly.
the wiring for them is fairly easy. there will be a buzzer that you can place wherever you like, id recommend putting it at the back of the car somewhere though as some of them are really loud.
the other wire will go to the live wire that goes into the reverse light bulb.
that way when the car goes into reverse, the parking sensor unit will become active.
the other wire will be a simple ground wire. either attach it to the body of the car or splice into another ground wire.
hope this helps a little