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).