An obi is the belt used to hold up kimono. (In this context.) There are over 300 ways to tie an obi, and each woman ties her obi slightly different. The most popular way to tie an obi is called otaiko musubi or "drum bow". It is most widely used with Nagoya obi, because they are light and very easy to work with. You can use any obi you like, but try to get one of Nagoya length (11.8 feet) or standard length (between 14-15 feet) to start with.

Before you start to put on your obi, you will need some accessories. You don't have to use all of them if you are just wearing a casual kimono.

Obi Accessories
These are listed in the order you will need them!
1.datejime: This is also worn on the nagajuban (under kimono), but you will need another one.
2.obiita: This looks similar to a cloth weight belt. It is buckled around the datejime (under-obi) to prevent the outer obi from wrinkling.
3.koshishimo or karihimo: (You'll need 3 for the drum bow.) A long strip of cloth that is tied around the waist during the tying process to prevent the obi from moving around.
4.obimakura: An oval pad with 2 long strips attached. This is put underneath the obi and tied to make it look fuller.
5.obijime: A braided cord that is pulled through the bow and around the waist to hold everything in place. They vary in length and color, so find one that suits you.

Okay now, here we go. Brace yourself.

Putting on the Obi
1. Put on the datejime just like you did with the nagajuban. Then put the obiita on over it and buckle it so the big part covers your stomach.
2. Fold the obi in half with the open side up. Wrap it around your waist once.
3. Bring the left end of the obi (tesaki) to the front. Pull on the other side of the obi until the tesaki length is about the width of your waist + an extra 6 inches. (It's easier to start with about 8 extra inches, until you get the hang of tying the bow.)
4. While you are holding the tesaki with your left hand, wrap the obi around your waist again. Then pull the tesaki out and wrap the obi again.
5. Pull the tesaki around your left side and to the center of your back. Let it hang down over the obi and hold at a slight angle.
6. Wrap the right side of the obi (tare) around your waist again. Fold it UP over the tesaki, but don't cover it completely.
7. Slide the karahimo (long strip of cloth) under the fold you just made but don't tie it over the tesaki. Wrap it around your waist twice and tie it in the front at the center.
8. Bring the tesaki to the front and hold it with the karahimo.
9. Unfold the tare in the back and tie another karahimo around it and tie it.
10. Lift the tare off your waist, leaving about 3-4 1/2 inches of material hanging down your back, and tie the last karahimo there.
11. Place the obimakura under the back of the obi with the flat side of the pad to your back. Wrap it, and tie it on the left front side once, tucking the ends in the obi.
12. Untie the karahimo ONCE that you tied in step 10, and tuck the last part of the tare under it. Once you are satisfied that the "drum" looks right, tie the karahimo in the front again.
13. Now remove the karahimo from step 9 and bring the tesaki to the back, slipping it through the "drum".
14. Take the obijime and slide it through the drum, tying it around the center of the tesaki and obi. Tie it once, but don't knot it.
15. Make a loop in the obijime on the right-hand side and tuck the left-hand part through it. The finished knot should resemble a smooshed fortune cookie shape. Tuck the ends under the obijime Remove the other 2 karahimo and you're done!

Though it seems complicated, this is one of the easiest obi bows to tie. It may take as long as 45 minutes the first time, but once you get a grip on it, you can easily do it under 10.

WHEW!

Please note that depending on your marital status, you may need to tie the obijime lower or higher.
Thanks to http://www.info-niigata.or.jp/~rikaboo for the correct name for the bows.