Quite easy, undo the two screws (nuts behind) that secure the slatted grill, this drops out, if a newer car there are lugs to fit the stone guard to and these are provided on the guard, my car a '99 does not have the provision for the lugs.
In my case by using a cable tie to hold the mesh together with the grill the assembly is slotted into place and held there, in fact the two holes provided at the bottom of the mesh are too high so the mesh actually sits on top of the screws that secure the grill.
Whilst out gave it a birthday and using some black paint tidied the nacelle area, added stainless tube to the bumper bracket etc.
Job took all of 15 mins.
If I can do it I will post a photo of the effect this weekend as I am away just now from home.
)