I bought my OEM deflector as a kit and had the dealer install it. The kit came with (1) the deflector (2) two brackets that screw onto each roll hoop (3) new roll hoop covers with pre-drilled holes (4) two inserts for the roll hoops and (5) two brackets for storing the deflector on the underside of the boot lid.
In order to install the inserts, you have to remove the roll hoop covers. That will likely damage the covers or break the brackets holding them together. That's why the kit has new covers. I would be very wary of removing the covers if you don't have a replacement set like I did.
Also, I would test fit the thing before drilling any holes in your roll hoop covers. Pop the brackets into the deflector and test fit it between the roll hoops and mark where the brackets fit snugly and properly against the roll hoops. That way, you should not drill the holes in the wrong place.
There used to be a very nice instruction with pics on another forum. If I can find it, I will post a link.
For the kit: http://www.unofficialbmw.com/images/6258EN.pdf
Ah ha! Found it!! Voila: https://www.bimmerforums.com/forum/showthread.php?1226398-Factory-Hard-Wind-Blocker
The good stuff is at Post #11. Lots of pics for you to see what is involved.
Aso, see the comments of "Chickdr", particularly at Post #18, 31 and 33. I think his situation is the same as yours, so you can read about what he did to install his deflector.