I made Alma's Halloween costume this year. It started out a fairy princess, but she declared it a Tinkerbell costume, so Tinkerbell it is.
I started with Wee Wonderful's Fairy Dress-up tutorial, where I got the front of the shirt (gathers, trim, etc.) But since it's October in Massachusetts, I went with velvet and long sleeves (I used another knit shirt as a pattern for the sleeves).
For the skirt, I had a lot of chiffon and other shimmery silky fabrics in the pile, so I cut out three circle skirts (two chiffon, one pants lining polyester) and a pile of petals for the waist. The shimmery petals looked great, but are really prone to unravel despite (or maybe due to?) the serger's rolled hem, so I don't know how long the petals will last. The waist is put together a little clumsily, as sewing all four layers together got tricky (and also: my sewing machine rebelled and I had to get out my old Singer. And spend about an hour searching for a bobbin for it). But there's elastic and snaps holding it together.
I debated the waist area for a while. I didn't want to tuck the shirt in--I thought it might look sloppy given the crazy snap/elastic thing on the skirt. So I cut the shirt to drop to a point on the center-front and -back. And then sewed in more trim.
She had some butterfly wings already, but they were held on with white elastic, which would totally ruin the front gathered detail of the costume. So I put some snaps on the back of the shirt and added snaps to the wings, so they could attach to the shirt. This was a little sloppy, as I cut the shirt big enough to last her a year or two, and the wings then made the shirt slide down in back.
Anyway, she didn't mind at all. She was happy to wear pajamas underneath (it was a cold night). She wanted a ponytail, ala Tinkerbell, but when it came time to brush her hair into one, she opted for hair down. And she had a tiara and wand that of course had to be part of it. Voila!