I haven't a clue what your bike looks like, but if it's anything like mine it has wheels that sit in forks with little holes in miscellaneous locations. Your challenge is to arrive at a shape that fits your bike and keeps liquid substances (icky or otherwise) off of your impeccable white tuxedo. Or am I the only one wearing that on my bike?
Step 1: Design, Materials, Tools
Bike fenders can be made from a staggering variety of materials. Just look at this:
In fact, you could probably do a web search for any noun plus the words "bike fender" and you'll find something. Go ahead and do that now, I'll wait.
So, what will it be? All form? All function? So beautiful that it brings a tear into the eye of the crankiest engineer and most jaded artist? So revolting that traffic around you parts like the Red Sea as drivers pull over en masse to vomit?
See doodles. For this first try I settled on a pointy-at-the-ends and narrow-in-the-middle design. I drew quite a few others though, and now that the concept is proven and I discovered how easy it is to cut bucket material, I may crank out a few weird shapes that I can swap out from time to time (think squid, ferret, snake, lightening bolt, cloud). Or maybe something plain and rectangular for formal occasions?
The most restrictive point is where the fender has to fit between the forks so you will probably need to go narrow there, but apart from that there is really no limit to what shape your fender can take! The front fender is usually shorter than the back, but even that's not set in stone. Look at tried-and-true fender designs, then figure out your own!
For mounting, I'm using only two rods instead of the customary four that you usually see in bike fenders on the market. This seems to work well after the testing I've done, and hopefully I won't regret the choice someday as the rod jumps between my spokes and I go careening over my handlebars.
I happened to have a partial 5-gallon bucket - the upper half - left over from another project. And also a surplus of bucket handles (because we used a bunch of buckets for chicken nesting boxes, and the chickens didn't need the handles due their lack of opposable thumbs and tiny brains). So because it's hard to argue with a price tag of $0, those are the materials I settled upon.
So, here's what you'll need:
-Plastic bucket: buy new if you must, but it's way cooler to get one for free!
For this project you'll need only one bucket, but four bucket handles (you can use other stiff wire for the support rods, but sticking with bucket parts is more cosmically harmonious).
-Zip ties: they hold the planet together, going where duct tape cannot.
-Something inky that can draw on a bucket (I used a Sharpie marker).
-Something sharp that can cut a bucket (I used a box cutter).
-A couple of pinchy things to help you bend a bucket handle (I used a pair of pliers and a pair of vice grips and a bench vice and, briefly, a hammer and some profanity).
-Something pokey or drilly for putting mounting holes in the bucket material (I used a drill).
Step 2: Cut It Out
Slice the bottom off of your bucket, and the ridged top section. Then slice the bucket down the side and "unroll" it so you end up with a flat, curved rectangle. Or if you're going with longer shapes, you could slice in a spiral so you end up with a long rectangle.
The plastic wants to curl back into its former shape so it will fight you; deal with it. Or shoot it with a heat gun to relax it a bit.
Cut out your shape. One bucket can yield several fenders, so don't fret if you botch the first one. You can go ahead and start fretting if you botch the second and third ones.
Now hold it up to your bike. How did you do? Good? Hey, congratulations!
Now drill holes wherever it makes sense to do so: I only drilled a pair of holes in the middle for zipping to the front fork (another pair where each rod crosses the fender is a good idea, but you won't know exactly where to drill until the next step).
Step 3: Bend Your Rods
You're shooting for the shape in the drawing: one end left in its original, wiggly state, then a length of nice & straight, then a bend that doubles back into a "clip" for the fender.
You may need more than two hands for this part, so perhaps you can get some kind of cybernetic implant. Barring that, you can zip-tie the fender to the fork at this stage, before you figure out the rods.
Hold the fender vaguely in place, stick the still-bent end of the mostly-straightened bucket handle in a hole near the hub of the wheel. Angle the rod toward the front of the fender (where it might be once it's installed), and figure out where the wire hits the fender's edge: that's where you'll be putting your next bend.
Convince the wire to turn 90 degrees (over the tire). Then measure about 1/8" wider than the width of your fender, and bend back, 180 degrees. This forms the slot where the fender is pinched into place (see photos).
Make a slight upward curve where the horizontal wire reaches the vertical one, and cut (I did that by bending it back and forth until it broke, since I was already holding the pinchy things and didn't feel like finding something cutty). This curve is where a zip-tie will sit, holding the fender in place (see note above about seeing the photos, then see the photos, as per the note).
Do the same thing for the other side, angling the wire toward the back of the fender.
Step 4: Zip It Up!
Place the bent end of the rods into the holes at the hub, slip the fender into the angle slot you made in the rods. Zip-tie the narrow point of the fender to the middle of the fork, zip-tie the rods at the hub, and zip-tie the fender into its slot. With zip ties.
Adjust the angle of the rods, adjust the slot so it holds the fender more tightly, adjust the location and/or number of zip-ties for security.
I noticed that the ends of the fenders were wanting to curl quite a bit, threatening to hit the wheel. I heated the fender a bit with a heat gun and moved the supports closer to the edge.
Once you figure out the right angles, it's a good idea to drill a hole on either side of the rod where it hits the fender: that would completely prevent slippage. I placed the word "finalize" in "quotes" because zip ties are easy to remove and replace (especially if you go with removable ones), so there's nothing "final" about this project - it's easy to keep tweaking and hacking and fussing with new designs.
Be sure to cut away the ends of the zip ties. Leaving the ends all pointy is a step into societal degradation that we just don't need to take.
Step 5: Keep Rolling...
Note: As of the publication of this 'ible, I haven't done my back fender. But luckily now I have this nice Instructable to show me how. Although I might not need a back fender too badly, since my bike now has a bucket on the back to protect me from icky liquids. Knowing me as well as I do, it's probably more likely that I'll make more wacky front fender shapes before I get around to the back fender, but I'll just have to wait and see what I end up doing. When/if I do get around to it, it will look something like the first scribble in this step.
Other ideas that didn't make it into the scanned scribbles: I bet if you lay a strip of plastic onto an (old, scrap) bike tire and hit it with a heat gun, you could slump a nice arc into it, so you wouldn't be stuck with flat designs. You might need to make a few ripples or slices or overlaps to get a nice tire-fitting curve, but I bet its within reach. And once you have the heat gun out, maybe your wacky squid-shaped fender could have 3D undulating tentacles? Or multiple tiny bends would give you a wacky corrugated effect. Or you could put a twist in the middle. And we haven't even considered the possibilities of color: paint? Glow-in-the-dark paint? Reflective tape? Faux scrimshaw (scratch designs, add black ink, wipe excess)? Colored buckets (they are available)? Covered with crochet? Covered with parachute cord? Covered with frosting? LEDs? The mind reels, does it not?
Now, go put on your tuxedo and ride your bike!