http://sewingmamas.com/b/downloads.php?do=file&id=414 for their helpful explanations. How does the saying go? "If I have sewn further it is by standing on the sewing machines of others"? Something like that!
And Kerri (@sewmamasew) is absolutely right about polyester chiffon - it frays and it will drive you mad. Remember this when you are at the fabric store. BUY THE NYLON! We forgot and had to overlock most of the skirt....
But it was Sandrahd (@sewingmamas) whose photos made it all make sense. I think I have a visual learning style. For anyone else who needs to see a pattern or a diagram, here goes:
The waist piece (in a satiny fabric) will be, in the finished skirt, one third its length. As you are making a double skirt (with inside layers and outside layers), the waist will be twice one third (ie two thirds) times twice your skirt wearer's waist measurement. Most preschoolers will have about a 20" waist, and twice this is just less than the roll width of most cheap satins (105cm*), so to make your life easy, just cut a piece selvedge to selvidge. Similarly with the layers - you will need at least 18 pieces one third the length of the finished skirt, and the same width as the waist piece - and the ONE GOOD THING about polyester chiffon is that it is easy to tear in a straight line selvedge to selvedge, so it was easy to create the 18 pieces.
This is what it looks like laid out - in diagram, and real life:
The more pieces you add, the more full your skirt. Or, you can add more to the bottom layer for more "pouffiness".
Here's the 18 pieces laid out on the floor. The bottom tier is bunched up to fit in the photo.
The best way to work is to cut all your pieces first, then work from the bottom up. It is much easer to sew each layer onto an ungathered layer, above it, than to sew it togther when all gathered.
First, gather your "fluff", or the frill for the bottom. You will need two or three times the length of your bottom layer, cut into 5cm or 2 inch wide strips (or buy pre cut strips in rolls). Stitch along the middle of the strip with a gathering foot on your machine, or a long stitch if using an ordinary foot (but try turning the tension up and it may gather for you). Pull the threads to gather, but don't gather too tightly or it will be difficult to sew onto the skirt.
When you have a mountain of gathered fluff, stitch it onto the bottom of the bottom layer of the skirt. Don't cut the fluff if it is longer than your bottom pieces - just keep feeding new pieces in until they are all joined by the fluff.
Next, run a gathering stitch along the top of your bottom layer (or use a gathering foot) to gather. Now you'll need to remember your maths. Gather it tightly enough so that the gathered tops fit the (ungathered) bottom of the layer above. So if you are sewing two pieces onto one piece in the layer above, gather each of the bottom layer to half its length.
Sew the bottom layer to the top (ungathered) layer.
In this photo, I've gathered the bottom tier and attached it to the top tier of chiffon. As you can see, I gathered each piece seperately. As I was gathering by hand, it was easier to keep it in manageable chunks, then sew it all together at the end. With the mountains of gathering you are creating, you wont see the joins.
Next, gather the top of your first tier of chiffon. Again, once gathered, the lengths should add up to the length of your satin layer.
All the gathering is done, and the next stage is to join the ends of the satin, so you end up with satin tube with frills.
Then, fold it in half (as indicated by the pressed line, below). Create a casing for the elastic, add a bow, and it's made.
Other websites have a lot more information about the process and the amount of fabric required (at least 4m for a 12" long pettiskirt, I'd say), but I hope that this is helpful to someone.