In most cases, cars require bicycle racks that attach to the vehicle's boot. While installation specifics vary from model to model, the general approach is consistent between most boot-mounted car bike racks. When you properly affix a bike rack to your car, you should be able to rest assured that the rack -- as well as your bikes -- will stay put, even when driving at relatively high speeds on the motorway.
Adjust the legs of your bike rack to fit the contours of your car. Most racks have a pair of arms that rest on the top of the vehicle's boot or at or near the bottom portion of the rear windscreen. One or two lower legs typically rest on the car's rear bumper. In some cases, the rear arms rest against the portion of the boot that is perpendicular to the ground.
Tighten the mechanism that keeps the legs in place once you have put them in the desired resting position on your car. The Saris Bones series of boot-mounted racks uses knobs that you hand-tighten to keep the legs in place. On most types of car bike racks, you need to tighten some type of knob, screw or bolt. Consult your manufacturer's instruction booklet for specific guidance.
Secure the hooks at the end of the straps that hang off of your bike rack into the grooves of your boot. The Saris Bones attaches to the car in this way. Most other boot racks use this or a similar method. In the case of the Bones' racks, two hooks are marked "top," two are marked "side" and two are marked "lower." You place these hooks in their respective positions along your boot. It is common for some model's lower hooks to attach under the rear bumper or underneath the vehicle.
Pull the strap that corresponds to each hook tightly to tighten the hook's hold on the grooves of your boot. Begin by slightly tightening each strap until the rack does not fall off when you let go of it. Tighten each strap several more times until you can no longer pull the straps through. You know they are pulled tight enough when your entire vehicle moves when you shift the rack's arms (the bars that stick out to carry your bikes) from side-to-side. When you do this, the rack should not move; rather the rack should effectively be one with the car.
Follow your car bike rack's instruction manual to place your bike or bikes on the rack. The Saris Bones, for instance, has separate positions for each bike you intend to carry. You simply place each bike's top tube (the portion of the frame that goes from the seatpost to the handlebar) over the arms of your rack and fasten the straps at each position to secure the bike. This method is common. Variations to this method are not drastic, though some racks require the use of bungee cords or some other device to keep the bikes secured on the rack.