Um....the best way to go about this is:
DONT INSTALL A 4 POINT HARNESS IN A STREET CAR!
It is VERY dangerous to use a racing harness for everyday driving. Also, were they made by a real company or did you get them off ebay? If you got them off ebay, you might as well not even have seatbelts in your car.
Now if you still insist on installing them, you will have to mount them to a roll cage or a bar behind the seats. You may also have to drill out holes to mount the harness at the bottom.