The pictures tell the story; The boot was an old German Paratrooper boot with the sole missing but I think any old boot will do as long as the leather is still good and the boot is large enough.
You will need;
Your bicycle complete with its mudguard
An old boot
stout paper or card for the template
a drill and drill bit to suit the rivet size
A hand operated riveter and rivets
Dubbin or Nikwax
How to do it;
make a template,
fold it in half to ensure it is symectrical
offer it up to check it looks right on the mudguard, adjust as necessary
cut the leather (I have heard that you should soak the leather in water to make it easier to cut but I had no difficulty with ordinary scissors)
treat it with wax or dubbin on both sides,
drill and pop rivet it to the mudguard.
I used other scraps of leather for hub cleaners - but that is another instructable...
Finally go for a ride and enjoy it - but do look where you are going not at the mudflap -I accept no responsibility for riders colliding with milk floats or street furniture!
This is the link to the bicycle mud flap on Flickr
Enjoy; Ride free; keep old bicycles on the road