A fundoshi is simply a 3-meter (or 3-yard) strip of lightweight, breathable fabric that is wide enough to cover your privates when laid flat in front of you. Some people like a narrower fundoshi, some people like a wider one that will gather a bit and have a pleasing "crinkly" look. Personally, I like my fundoshi about 9-10 inches wide. When all is tied and done, this is much narrower in the front, maybe 6-8 inches across -- the tightening and snugging of the fundoshi as you tie it causes it to compress in width a bit, which forms a natural "pouch" and that I find pleasing to the eye and comfortable on the body.
It's pretty safe to experiment until you find what's just right for you -- fabric is cheap and a fundoshi is almost infinitely adjustable for snugness, where it sits on your hips, etc. Others have compared the fundoshi to "fabric origami," and that's an apt comparison -- it's an elegant design solution to covering and containing the male (and female) private parts, and keeping them clean and safe from chafing etc.
On the most basic level, that's probably why the loincloth (of which fundoshi is just the Pacific islands' variation) was the very first piece of clothing to evolve: the "most basic marker of civilization" according to Otto Steinmayer, and the world's oldest menswear!
Almost every culture has some form of loincloth, from ancient Egypt to Native America. Boxers and briefs are a really, really recent thing, historically speaking.
Here are some diagrams showing how to tie a fundoshi that you might find helpful:
(This first one is from a commercial fundoshi package, scanned or photographed in the early days of the internet it looks like. But it's one of the first really helpful diagrams that I found when I was searching for fundoshi-related things in my twenties.)
(Here's the directions in English. Click on the picture for a larger view, so you can read it easier.)
(it's from some sort of Japanese variety show??)
Finally, a few tips:
1. It's a good idea to keep things snug as you go. Too loose isn't really comfortable, and is only suited for lounging or maybe sleeping due to increased risk of "wardrobe malfunction."
2. Practicing is actually fun, don't worry if it takes a while to get it right. Use a mirror to help you follow along with the steps. After a few times, the mirror won't be necessary.
4. A well-tied fundoshi stays on no matter what, until you or someone else unties it, even if you're swimming or being active.
5. Look at pictures and diagrams to help you get the shape right. There was a time when the same 12 fundoshi pictures were all that ever turned up on an image search. Now, there are thousands and thousands of fundoshi photos, from vintage to present.
6. There are hundreds of regional variations on how to tie fundoshi, mine is just one simple way to tie rokushaku-style that I was shown -- feel free to experiment and find what is most comfortable. Since fundoshi holds the genitals close to the body, I recommend wearing it for no more than 8-10 hours at a time. Same as you would wear a swimsuit, typically. For cold weather though, fundoshi makes an excellent underlayer.
7. Although white is by far the most common color of fundoshi, ANY color or pattern is possible. Bright red fundoshi, often used for swimming, have their own name even -- "akafun," which simply means red fundoshi. So have fun at the fabric store!
8. If you're worried about the "wedgie" factor, I can assure you that fundoshi are quite comfortable! It only took me a few minutes to get used to the new sensation of wearing a fundoshi, and now I wear fundoshi all the time -- either by themself or under my regular clothes. I've grown to really like the feel of a nicely-tied fundoshi. I bet you will, too.
Thank you for visiting this site, and best of luck exploring fundoshi, your new favorite thing to wear! (...what are you waiting for? Go get some cloth and try it out!)
Feel free to contact me via Google+ if you'd like any advice or feedback related to fundoshi-wearing.