Step One: Strap rack to car.
Step Two: Strap bike to rack.
And there you go. Strap racks are a simple way to carry bikes behind you for infrequent long-distance carries or frequent short distance carries. For longer distances or frequencies, you'd be happier with a hitch rack. It's a lot easier to get in your trunk that way. But dollar for dollar, strap racks do the job. Super cyclist who carries his bikes 24/7: hitch rack. Folks who take a few bikes to the cabin up north once in a while: strap rack.