First remove the black plastic rail from the frame (you can hook your nail behind it and take it off).
Then you use WD40 to spray some grease between the frame and the small rubber part on top of the rail which holds the cable of the brake on the right hand side (you can use a small flat screwdriver to get the grease between the rubber and the frame, be careful for scratches!).
Put a non sharp object underneath it (like a 5 hex key) and slowly push the rubber part out of the frame. It's hard to do. If it's off, you can slide the bottle holder into the frame. Screw it where you want to fix it. Is it fixed? Put the rubber part back. After that, cut the black plastic to the right size top and bottom and push it back into the frame.