Assuming you are talking about a roof rack: 1) loosen the bolts that secure the bike rack to your car. You will need a wrench or socket set for this. 2) remove the front wheel of your bike and place it in one of the wheel wells on the rack. 3) do the same with the rear wheel, being careful to line up the cassette with the frame mount. If your bike has disc brakes, there should be an adapter on the rack to accommodate this. 4) once both wheels are secured, tighten down the bolts on the rack. Make sure they are tight enough that the rack won't wobble, but not so tight that you strip them.