This is John from Holdcrofts reply to me when I asked....
The flaps are mainly held on using the existing holes which the fastenings for the Wheelarch shields fit . These are replaced with the ones in the mudflap kits so the go through the flap and the Wheelarch shield into the same hole you took them from . The drill refer's to a small pilot hole so the support screw can start as it then cuts its own thread .
hope this helps
JOhn
Edited by: Fontie