There are literally hundreds of ways to tie rokushaku fundoshi, that vary from region to region and sometimes city to city. Here is a good simple diagram.
And here are the basic steps:
1. Hold about 1/3 of the cloth at your waist so that it falls about to your mid-calf/ankles.
2. Now flip this 1/3 over your shoulder (or hold it in your teeth). See the video below.
3. Pass the other 2/3 back between your legs, then bring it all the way around your body from back to front and back again.
4. When you get to your back, loop it under the first part and draw it tight. For now, tuck this end under the waistband you've created.
5. Now take that first 1/3 you had over your shoulder and let it fall. Draw this back between your legs as well.
6. Cross it over the tucked-in loop (which you can pull out now), and wind both ends around the waistband, in opposite directions. These two ends should end up about the same length.
7. You're done!
If I could offer a few tips, they would be:
- it's a good idea to tie your fundoshi tightly. Obviously not too tightly, you don't want to cut off circulation down there! But a well tied fundoshi simply will not come untied, and will remain comfortable and supportive all day, even if you are being active or swimming. In fact, to "tighten one's fundoshi" is an expression meaning to focus one's mind, as if for hard work or battle.
- A soft, thin, breathable cloth is best. My first fundoshi was an old, white bedsheet which had been laundered many many times and it was quite comfortable! Cotton or muslin seems to work best. A soft gauze would also work. Thicker fabrics are too bulky, and slicker fabrics like silk don't stay tied very well. You can easily make your own by tearing 3 yards of fabric into 10-14 inch wide strips. No scissors necessary, no hemming. These raw edges will get softer (and fray less) every time you launder your fundoshi. Happiness is a warm fundoshi fresh out of the drier.
- American Apparel sells a cotton jersey scarf that is almost the perfect dimensions for fundoshi: http://store.americanapparel.net/6445.html#i
Here's a video that makes everything a little more clear:
I don't tend to twist the waist band like the guy in the video does, that way it lays flatter against my stomach and is a little more discreet underneath street clothes. It's a matter of taste really. With a little practice (and practicing is fun!) you can wear fundoshi daily.