Auberginefleur is the author of the blog Japan Now & Then. She blogs about real world events, and places she visits in Japan, and she also blogs about wearing yukata.
I thought that Auberginefleur did a great job describing the pieces to the yukata in a way that most would understand (in particular me *smiles). Auberginefluer not only posts about yukata, she also wears them on various occassions (see her other posts). Her post, separated the yukata discussion into 7 parts; hada-juban, yukata, date obi, yukata obi, koshi-himo cord, obi-ita, and geta.
The following is a quoted from the Aubergineflueur's Auguest 5, 2007 blog post in Japan Then & Now:
How to Wear Yukata I: What You Need
1) One-Piece Hada-Juban for Yukata
This is worn over your underwear to prevent the yukata from sticking to your skin, and though I went years without bothering to wear one, it is much cooler if you do. The best is a 100% percent cotton or linen full-body shift. Avoid ones with a polyester skirt. Sales assistants will try to talk you into a large size if you are a gaijin to move the merchandise, but unless you are large, get the medium because the extra material of the skirt will make it hard to walk because it tangles around your legs. Appropriate underwear is a sports bra and panties for a woman and V-neck T-shirt and boxers for a man; men do not need a hada-juban.2) YukataThat goes without saying, but you put this on over the hada-juban. Do make sure you wash your hands before dressing, because make-up, hairspray, and other soil will stain the yukata. Hold out the sides of the yukata as you face the mirror. Line the vertical back seam with your back-bone to center the yukata.Raise it off the floor so the hem is just barely above your ankle bone. The lower the hem, the more elegant; the higher the hem, the easier to walk. Close youryukata right under left. Make sure the inner left hem does not show. Tie shut with a Koshi-himo cord at your waist.3) Koshi-himo cordThese cords are used to tie shut the yukata and also if you need to hold something in place while you adjust your yukata or obi. Technically, you only need one for a yukata, but I would recommend buying at least a pair, if not two pairs. They come in very handy. You can keep an extra in your sleeve in case of emergency when you are out and about. The matching belts that come with inexpensive yukata bought at Asakusa or the airport, are basically koshi-himo's.4) Date-Obi
This is a thin under-obi used to even out your yukata and fasten it in place for tying your obi proper. You can go without this and just use another koshi-himo instead, but the obi proper will be easier to tie and look smoother if you use a date-obi. Once you have fitted on your yukata to the proper length and tied it shut with the koshi-himo, stick your arms in the hole under the sleeves and push out the extra material to fold over the koshi-himo. Try to make an even hem-line at the waist. Now is the time to pull down the collar in back so it is not flush against your neck and make sure your yukata is closed properly at the breast. For first timers, I recommend a you now tie your yukata shut with another koshi-himo just under you rib-cage and do any final necessary straightening out. Otherwise, you can go directly to the next step of tying on the date-obi. Hold the date-obi so the center fold is aligned with your belly-button. Cross the two ends in the back and fold one side down so it folds smooth. Then pull the ends to the front and tie the date-obi in a bow in front, straighten out the bow, and then tuck the bow into the date-obi. Men do not need a date-obi.
5) Yukata ObiThe Obi for a yukata is half the width and also shorter thana kimono obi. It is often also called a han-haba obi or a ko-fukuro obi. There are basically no rules about matching a yukata obi with a yukata, so choose whatever one you like. The basic strategy is either to go with the same color scheme or choose a color in direct contrast. Unlike Western fashion sense, you can put different shades of the same color together with impunity. If you have a fancy koubai or ro-gauze weave yukata, you can dress it up by wearing a Nagoya obi, but then you start to get into the rules of proper kimono
The obi-ita is not strictly required to wear with a yukata, but it keeps the front of the obi from wrinkling. It is placed between the two layers of the obi at the front. There are types that tie on, so you wear wear it over a koshi-himo in place of a date-obi. Men do not need a obi-ita.
Geta are the wooden sandals worn with yukata. Men can wear geta or the flat straw sandal called setta.
Please visit her site for more links and information. She has a great post about yukata textiles. I hope you find her blog as enjoyable as I have!