Step 1: Materials
Bicycle that you want to put fenders on
One worn out wire bead bike tire wider than those on your bike
Some steel (or other metal) rod or tubing
A welder or torch to solder with
Some zip ties
Some scrap steel or copper sheet (20 Ga or so)
A power drill
A metal file
Some spray paint
A method to cut your tire
Misc bolts and nuts to mount your fenders.
To start with, take your scrap tire and cut it into sections that cover the desired portions of your bike wheels. See below for an idea of how much coverage is needed to keep you dry.