My experience when buying mud guards is that when you go to get them, you should buy the ones that are made specifically for you model vehicle, and not the generic ones they say will fit your vehicle, usually labeled as "universal fit."

I have a set of impreza mud guards in the shop
From what i remember they need to be drilled/screwed into the arch lip
Best way to do that is:
Apply masking tape on the lip
layout your hole locations
indent those spots with an awl
Drill slowly with a smaller than needed Sharp bit
remove tape
Apply light dollop of clear silicone (VERY LIGHT) on the screws threads 
Install mud guards

Hello!

Have you received any installing instructions with mud flaps?