Guess you didn't ask me.
On the bottom of the cover there is a metal clip that has a raised edge riveted to the cover. On the bottom of the plate attached to the car there is a clip with a tab sticking up that is bent toward the car body. Often the tab is broken off. You are supposed to be able to push the bottom of the cover so that it aligns and catches on the clip and holds the bottom in place, then buckle the top. A huge pain that seldom works (I re-made the body clip on my own car with larger tab so that it would actually catch the cover). Later cars just gave up and used a belt/buckle arrangement.