You drag and drop a photo file in the box, as instructed, and then start painting - red for remove, green for stay. Zoom in and use the eraser and smaller brush sizes to fine-tune things. The fourth icon on the left, the four-way arrow, lets you move around if you're zoomed in.
It just took me a few minutes to go from the picture on the left to the result on the right.
You can choose different colors for the background. Yes, a professional piece of software will handle the delicate tracing of Kinsey's hair somewhat better, and I could have invested a few more seconds in getting the right bottom area of her dress correct, but free and quick are both rather powerful concepts!
This service is currently in alpha testing, and is free during this time. There's no indication what the pricing might be after that, but you can sign up now and get freebies once the testing is over. In the meantime, go and play!