I found a long tulle dress at a thrift store and transformed it into this skirt.
Look for a fancy dress roughly the size of the child you are making it for. One that fastens with buttons up the back, and has a sash that cinches the waist are a plus. The sash isn't entirely necessary, but it makes it simpler. If you find a great dress with no sash, you could make a sash and attach it, or make a waist band from elastic - you would have to improvise something to keep the skirt on the child. Here is the one I found:
List of Materials:
dress (wash and dry it)
needle (I used a longish strongish one to sew through the layers, but I don't know that it matters)
good quality thread
relatively sharp scissors
pins
How To Do It:
1. Fold the bodice into the skirt, leaving a 2-3 inch "waistband". Make your waistband the size of the sash. See how it lays. This will tell you what cuts you need to make in the bodice. Mine laid quite nicely, so I knew I wouldn't have to make that many cuts, and could leave most of the bodice intact. I knew this would be helpful to prevent unraveling, and would add more body to the skirt, always a plus with a poofy skirt.
2. Cut your bodice through the top of the sleeve, as if you are cutting from the center of the arm through the shoulder seam. If your dress has long sleeves, I would recommend trimming those off. (You could use that fabric to make a sash if your dress doesn't have one)

3. Tuck the bodice back in and pin the waistband. I found it helpful to pin at the top and bottom of the waistband, so I knew it could lay half-way nicely once sewn. It does not have to be perfect. This is dress-up, and your kid doesn't care if it's perfect.
4. Form the waistband by sewing (tacking, really) around the bottom of the waistband, just above the skirt seam. I did mine every 1/2 inch or so. The picture below is the outside of the waistband. See how the needle is going back into the fabric close to where it is coming out? That's what you want to do on the outside. On the inside of the waistband is where you will travel the 1/2 inch. I like to sew with double thread, so that it's easier to knot the ends when you need to change thread. I'm sure this is "wrong" but it works for me (my mother didn't teach me this - I take all responsibility - Hi Mom! Love you!). This is the hard part because it takes a while, especially if you are making it for a bigger kid, because, bigger waist. But mine only took me 2 or so days to sew up - working on it whenever I had some free time.
You can see that my waistband is not perfectly perfect, but you absolutely can't tell when it's being worn.
After wearing it for a while, she asked me to tack the sash to the waistband in the back, because the back waistband would slip below the sash when she was dancing around. So I tacked it down 2-3 inches from the back opening, This helped keep the waistband up. No picture of this....sorry.
If you have any questions about a step, let me know.
One last picture for you of my ballerina: