There's a lip on it that slips under the seal. It's hold on pretty good with just that. But to be sure it won't fly off (and fly through the windshield of the car behind you..), you have to put a bit of sealant on the inside on the left and right. Just in de corner where the spoiler meets the window.
Here's a picture of two of them before installing:
The angle is aproxametly 12 degrees compared the rear windshield.
I was told that anything above 12 degrees becomes an obstruction instead of an airflow guidance.