The costume was put together in about an hour before an event (starting with the materials), so I hope you'll forgive the hasty photos.
Step 1: Materials
• A couple of yards of a 5-foot-wide bolt of deep blue fabric. I got way too much; three yards will more than suffice.
• A lime-green boa
• Five safety pins
• Assorted bobby pins
• Underclothes (boots and leggings are a good choice, as it gets chilly and you may want to bike around or dance!)
Step 2: The Robe
Start by holding one corner of the fabric in one of your hands. Slide your other hand down the length of the fabric, until your arms are fully extended. Since I'm 5'8", this gave me a rough square from shoulder height to the floor. You may want to vary the fabric width if you're taller or shorter.
Cut the fabric at that length.
Ignore the weird drawing (unless you really need it).
Step 3: Wrap!
Bring the sides, one at a time, across your torso and safety-pin the corners at a comfortable tension behind your neck. Cover the pin with the overhanging corner fabric. (Depending upon your bosoms, you may want to add further safety pins. I was fine without.)