Whats the best way to attach rear spats the after market one not genuine

They screw in or bond on

get ya self a rivet gun and ue some countersunk rivits then just touch up the paint

 

well thats what my mate did on mine and it looked the part

 

 

plus ya can always get them off if ya need to screws tend to rust up and bonding it means your pulling them off with force and thats not good IMO

i fitted some last week - i used some Sicaflex-221 (i think it was that) adhesive and two screws into the rear bumper at the arch - this way no screws show when looking from behind