Line the flap up where you want it mounted, and mark the holes with a permanent marker.
If you don't have a metal punch, and assuming you haven't installed the fender yet, have a friend hold the fender steady with the end over a wood block.
Use a sharp awl and a rubber mallet to pop a pilot hole in the metal.
Or use a very small drill bit and be extremely careful not to "walk" the bit when starting the pilot hole.
Progressively widen the hole by stepping up the bit size.
Use a round (rat-tail) file to finish the edges of the hole and remove any burrs.
Install the mudflap with the provided mounting hardware, or rivet in place. (If using rivets, back with a combination of leather washer against the fender to avoid rattling, and flat washer for the buck-end to catch against.)
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"I feel like my world was classier before I found cyclocross."
- Mandi M.