On this one you'll have to find a dress that thins out around the calves and ankles. You can alter an existing dress to make it thinner at the bottom by taking in the seam. Then add some shimmery fabric to the sides to simulate fins. If you are trying to look like Ariel, you will need to go with a green dress. Otherwise, any color will work.
For an easy top, you can dye a bra and cover the cups. Get a cheap bra and a pack of RIT dye. This dye is available at your local grocery store. Choose a color of the sea, like blue, purple or green. Dye the bra as instructed...here are some photo instructions for cold water dying.
Cut large scalloped shell shapes from a shimmery fabric (find inexpensive fabric online). The shells need to be large enough to cover the cups of your bra. Stitch these around the edges to loosely cover your bra cups. The top can be something like a bra if you want, but I have seen high-cut tank tops work just as well.
To look like Ariel, you'll need to have red hair. Don't use the spray-in hair color that you can find everywhere at Halloween. This stuff will easily rub off on your skin and clothing during the night. I suggest Ariel only for natural redheads, or for those who don't mind splurging on a cute auburn wig (see wig). Another way to do the wig is to make it completely out of red yarn with our yarn wig tutorial.
If you don't care to look like Ariel, any hair color will do, and it will help to embellish it with shells or flowered barettes (make flowered barrettes). String some pearls around your neck and add a flower to your hair. You can even find fake seaweed at the pet store that will stick nicely in your locks.
Light makeup is in order, again, and choose colors of the sea. I have seen some cute mermaids with starfish painted on their cheeks, glittery eyelids and soft blush to the cheeks.
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