an empty half-gallon (2 liter) milk carton (either with or without the capped spout)
a good pair of scissors
a permanent marker
hot glue gun or comparable adhesive
4 zip ties (available at your local hardware or electronics store)
When finished, the fender attaches only to the seat post stays of the bike's frame
Step 1: Open the Top
Open the entire top along the seam so that the top is completely open.
Step 2: Cut It Up!
The half with the spout (at right) will become the front half of the fender. This is the part that will wrap around and attach to your bike frame.
The other half will become the back of the fender.
Step 3: Back Half of the Fender
Cut a notch out of the bottom of the carton to allow clearance for the tire. This might not be necessary for all bikes but it doesn't hurt.
Step 4: Front Half of the Fender
Cut both corners so that the bottom is free from the sides.
Step 5: Attach the Two Halves
Withe two pieces laying top down, set the back half on top of the front half so they overlap by about 4 inches (100mm).
Mark the location and heat up the glue gun.
Apply glue to one mating surface of the carton and press the other half on top.
Repeat for each of the sides.
Step 6: Cut Notches to Fit Frame
These should be about 1/2 inch (12mm) wide and 3/4" (18mm) long.
Step 7: Pre-roll the Side Flaps
I used a sharpie to help shape the flaps.
Step 8: Put It on Your Bike
Holding the fender with one hand, make slits 1 inch (25mm) below the top and 1 inch (25mm) above the bottom for the zip ties.