Get out your utility knife...
You have to slit the inner lower flap, so that the part you hold can slip up inside the pocket in the flap. Drill the existing rivets out, so the panel is smooth and you can separate the two parts. There's two parts to each side, the main part that slips up in there and has the side lug attached, and a cover panel with two holes that shows on the inside, they rivet together through the inner fabric of the vertical pocket that the part slips into.
I think the one in your pic is for the other side, but check your owners manual and other sources for images to confirm side lug direction.