Step 1: Materials
Additionally you will need electrical tape or some zip ties or other method to secure the fender to the rear bike rack.
The figure below shows the kind of jug I used, along with a pattern that I used for the fender.
Step 2: Cutting the Fender
Next, draw your fender pattern on the plastic. The keys to sizing the fender is to make sure it is wide enough to cover your whole tire and long enough to keep spray from whipping off the rear of the tire up onto your back. My fender is 4 inches wide and about 24 inches long. This is pretty long as you can see in the pics. You can trim the fender afterwards if desired once the spray pattern of your specific tires is characterized. Spray comes off of your tires tangentially, so if you look at the layout of your bike you can see how long your fender needs to be to keep spray off of you in the saddle.
My fender (below) includes some notches on the sides for clearance of the side bars on my bike rack. My pattern also includes a pair of slots in the middle and a slot at the front to allow the fender to be taped and zip tied to the rack. Your pattern may differ and could include provision for mounting to a different type of rear rack, a support structure of your own design, or might include fancy scallops and designs for no good reason other than aesthetics.
Use your preferred cutting implement to cut the fender out of the plastic.
Step 3: Attach to Your Bike
There you have it! For a front fender you can use the same type of plastic taped to the downtube on your frame which will shield you when you are going straight (most of the time) or get creative. I have found that in most cases a front fender is not needed unless it is actively raining, at which point you are wet anyway. Going through puddles and other wetness can be handled by a rear fender only for the most part. As long as you aren't wearing a three piece suit or equivalent evening attire. Different tires pick up and spray water differently so as always, try it out and tweak it. Otherwise, you could try and build a front fender out of old tires, which is awesome.
With rainy spring on the way and melty snow in the road, your fenders will keep you from being a bike-skunk so get building.