If it's a factory steel car, the side "runners" have clip on one end that fits into a slot in the door and the other end uses a screw into the door frame. The piece that goes up and over the top is really held in place by the two lower sides keeping it up. I used some silicone adhesive to help hold it in place.

If it's a fiberglass body, it is still the same process except the 2 side tabs won't fit into the slot (there won't be one) so you'll need screws top and bottom and/or adhesive to hold it in place. HTH...