Skirt guards are hard to find in America and can be very expensive. Cable ties and scraps of tulle or other mesh material will keep your skirt safe without going to the Netherlands or haggling with a collectibles dealer. You can read about the inspiration for this project at www.evilmadscientist.com
Step 1: Gather materials
You'll need a bike with fenders, a handful of cable ties, and a couple of pieces of tulle or mesh slightly larger than your fender. A cruiser or touring bike will work best, but you could adapt this to a bike with a cargo rack or a seat post mounted fender with a couple of holes drilled in it. Tulle is cheap at any fabric/craft store (mine was on sale after halloween) or can be scavenged from old formal dresses. It is reasonably strong and doesn't hold dirt very well. You can spray it clean pretty easily, too.
Step 2: Prepare fabric to be attached to bike
Cut the tulle to a semi circle somewhat larger than your fender. The straight edge will be going from the rearmost fender stay to the front of your chainstay. Pick a spot close to one of the corners and stab a cable tie through. Tulle is tough, so push hard - don't be afraid.
Step 3: Attach to bike
Start on the non-drive side of the bike (usually left, but some tandems have both sides.) Wrap the cable tie around the fender stay and poke it back through the tulle. Attach the cable tie snugly to the fender stay and repeat on the chain stay so that you have the bottom line of the fabric going just above the hub. Remember, you want to be able to change a flat, so access there is important! Now attach the cable ties to the second fender stay and the seat stay. Once you get it all lined up, tighten all your cable ties well. How taut your skirt gaurd stays will depend on how tight your cable ties are, so tighten them very well - a pair of needle-nosed pliers can help with this. Now trim your cable tie ends.
Step 4: Drive train side
The difference on the drive train side is that you have to be cautious about not getting in the way of your chain. My chain guard has a handy attachment I used as a guide for where to cable tie it, but most modern chain guards won't have that. Aim for a spot just behind where your pedal travels. You've already got the chainguard (I hope!) to partially protect you. You're aiming to keep your skirt from wrapping around the seat stay or rear cog. Again, start with the straight line from chain stay to fender stay, and then add the seat stay and second fender stay cable ties.
Step 5: Make it all purty
After tightening and trimming the cable ties, trim the excess fabric from one connector to the next in approximately a straight line. It will end up with a nice almost-spiderweb shape.
Step 6: Cruise in safety and style
Check to make sure your fabric isn't loose or rubbing the tires (or worse yet, chain) anywhere. You are ready to roll!