The clips slide into slots on the centerline of the trim piece. The diagram only shows them like that for clarity. You will want the two-sided trim tape as well to adhere the top and bottom edges.
When you stick the trim tape on the back edges of the piece, don't pull off the plastic from the side facing the truck before sliding the molding onto the clips. Just stick one side of the tape to the molding, leave the other side covered. Leave a tag end of the plastic that coves the back side of the tape sticking out a couple inches. Put the clips in the holes, turn them 90 degrees so they grab. If I remember right, they are made to slide into the tabs on the molding one way, so make sure they are facing the right way.
Then slide the trim piece back on, making sure it engages all three clips. After it is in place, grab the tag end of the plastic on the tape, and pull the plastic away, exposing the adhesive to the door. This will adhere the two-sided body tape, and will keep the trim piece where it belongs.