In Canada, M-Performance parts can be purchased at our dealer and the dealer can install it.
I did have it installed by 3rd part bodyshop and from what they showed me it involves removing the entire bumper from the car then unclipping the original grey diffuser from the bumper. Then install the black diffuser and reverse the install.
I'm not sure if you can only remove the bottom piece without removing the entire bumper from the vehicle to gain access to the clips for the rear diffuser. Maybe...
Link here:
https://www.realoem.com/bmw/enUS/par...MW-330iX&mg=03