Easier to show than describe of course, but I'll do my best! (having spent about a year doing Iaido with the full gear now) This is approximately how my sensei told me to do it:
Start by rolling the obi up fairly loosely so that it's easier to deal with - you've probably noticed they're a LOT longer than other martial arts belts. Leave about 20-30cm of the obi loose. Place the roll on your left hip so the loose part is hanging down beneath the hip bone. Then wind the obi 3 times around your body in a clockwise direction, pausing at intervals to pull it tight - it needs to be tight enough that you can hardly breathe, otherwise the sword will be too loose. Now comes the bit that's complicated to describe.... The remainder of the obi should now be just in (to the front) from your left hip. Turn it over and down so it hangs downwards and makes a neat triangle against the already-wrapped bit. Then roll it up (really a set of folds, the same size as the width of the obi) until the rolled part lies on top of the triangle. Hold in place. Then take the other end (the one that hung from the left hip at the beginning), pull it slightly to the right so that it hangs below the folded section, then bring it up and over the folds, and tuck it underneath the whole thing (so the end bit hangs downwards again) and repeat until you have no more than an inch or two hanging down. Pull tight. Then grab the entire knot and rotate the whole obi clockwise until the knot lies at the small of your back. Finished!
Clear as mud? Like I said, easier to show than describe.
You might try checking out an Iaido website, such as Kim Taylor's in Canada (www.uoguelph.ca/~iaido/iai.sdk.html
) and seeing if there are any further descriptions, or maybe email him - I'm sure he's had to describe obi-tying many times. Good Luck!