I liked this t-shirt dress tutorial from Amanda over at I Sew, Do You?. In fact, I like her sample (orange and black) so much that I may re-create it exactly for Halloween. I think hers was in honor of her university's school colors, but it would be perfect for fall.
Amanda's tutorial is great---just follow the directions. I made everything in a little over an hour.
To give it a more modern look, I cut the dress longer on the sides and shorter in the front and back. I plan on her wearing this over leggings.
Amanda gives advice on using tissue backing to help you sew with knits. I didn't do that since my machine was handling the fabric OK. However, I know knits can be tricky on some machines, so give the tissue trick a try if you're machine is finicky.
For her bow, I used a simple stretch elastic band, then made a rosette of the leftover ruffled fabric. I sewed on a big pink enamel flower that was at the bottom of a garage sale bag of broken costume jewelry. Do you guys go crazy over a big bag of broken jewelry? I do--and I end up using the stuff for years.
So there---a quick, cheap, upcycled dress. Good for your wallet and the environment.