Tying your fundoshi takes practice. No one will say otherwise. Unlike "clip-on" neckties, there is no shortcut to a well-tied, comfortable, and body-flattering fundoshi. The good news is, practice can be lots of fun! First off, you start naked. Lots of good things start with nakedness.
Selecting the right fabric is part of the fun too. Something lightweight, airy, soft -- just this side of gossamer. Not too silky, as the slipperiness of silk fabric against itself will make tying very frustrating.
Once you've picked the right fabric and found the width you like (and I will always say, wider is better), learning to wrap a fundoshi really only takes a few minutes. But learning to wrap a fundoshi so that it stays secure, is not too tight or too loose, and so that it looks good on your body... that's an art.
Key to this is getting the back of the fundoshi right. This post will focus on that. While some wind the fabric, rope-like, around itself as they pass it between their legs and buttocks, this is really optional. The only essential roping of the excess cloth occurs at the final step of tying the fundoshi, when you wind the excess around the waistband you created with the first few steps.
It's important that the two "ends," the parts that you will wind around the waistband on your right and left thights, cross over eachother at the back.
If the two ends don't cross over eachother before you wind the excess around your waist, you'll get a "Y" shape in the back, at the base of your spine, instead of the desirable "T" shape. Play around with this idea, consult the diagrams and how-to videos in earlier posts -- you'll get it pretty quickly. It may take a few dozen times before you tie your fundoshi on like it's second nature, and so that it fits and flatters your form.
Fortunately, the feel of fabric tightening against your body is very pleasurable. You may find yourself untying and retying the fundoshi just for the sensations of restriction and release that your body will feel.