Okay here's the tutorial! It's my first one, so if you have any questions just ask.
1. The first step is to construct the underskirt. I used satin, you can really use whatever you want as long as it's opaque. Figure out how long you want it to be (I made mine a little above knee length) first, and then cut out the fabric. I used my favorite method for making a circle skirt which is this:
2. Since I was using satin, I immediately serged the edges, which if you have a serger and are using satin I HIGHLY recommend. That stuff frays like nobody's business. Then, you'll cut a piece of elastic a few inches shorter than your waist measurement, and a piece of fabric that's about 2.5 times the width of your elastic, and several inches longer than your waist measurement!! This is important, because I was a ditz and made it only an inch or two longer than my waist measurement, and now it will only fit over my boobs if I'm not wearing a bra, and won't go over my ass at all. Just sayin'. Keep it in mind. Then you'll fold the fabric over, stitch along the edge, turn it inside out (or not - I just sewed wrong sides together since the bottom of the elastic is under the skirt anyways, so I saved myself a step), and put the elastic through it. It might take a while. I'll wait.
3. Now, you'll pin and sew the long tube you just made to your circle skirt and stitch! I just left the hem of the skirt serged and didn't officially hem it because a. I am lazy and b. it's under a freakin' pound of tulle anyways, so nobody knows but me.
4. Here comes the fun part. THE TULLE. Woo-hoo. Okay, you'll cut out two pieces of tulle TWICE as long as your waist measurement and several inches long (depends on how many tiers you want to do - I did three tiers, so I my top two tiers were 7" long and the bottom was 8.5", as you saw, this resulted in a skirt a little longer than knee length). There can be several variations from what I did at this step - if you'd like, you could do three times your waist length for a really poofy tullemonster. Or sandwich in a layer of lace betwen the two layers of tulle for some added fanciness and opacity, or just do another layer of tulle. And of course it's possible to vary the length of the tier. At any rate, now it's time to run it through the sewing machine on a basting stitch - I used a 4mm stitch. Leave longggg trails of thread at the end. Now go look at one of those ends, and tug on each of the threads. One of the threads should start gathering the fabric when you tug on it. Tug and tug and tug some more, even out the gathering as you go, until the gathered edge of the tulle is as long as your waist measurement. Set this aside for now, someplace where the damn cat won't get ahold of it and break the threads, undoing your hard work. Luckily for me, he only got the last four inches or so.
ALTERNATELY: If you are a lucky lucky person who owns a ruffler foot and knows how to use it, just fiddle around with the settings until you get a setting that gathers to the needed amount (i.e., can take a 10" strip of fabric and turn it into a 5" strip of ruffled fabric). Or you can do as I did, which involved having trouble with the ruffler foot, swearing loudly at the sewing machine, and grumpily resigning yourself to hand-gathering. Whatever works.
5. Now, take the length of the last piece of tulle that was cut cut, pre-gathering, and double that (we'll call the resulting measurement x). Cut out two 7" by x" pieces of tulle, and do the basting stitch along one edge again. Now, you'll gather it to the original length of the first piece of tulle. This would probably be a good time to put on some Buffy or something, because it could take a while. Once you get done gathering it, pin it and sew it to the bottom layer of the first tier.
6. This time, you'll take the length of the second tier pre-gathering and double it (measurement x). Cut out two pieces of tulle that are 8.5" by x", and baste one edge. Gather it until the gathered edge is the same length as the pre-gather length of the second tier (this will take a WHILE - I reiterate my suggestion to put a dvd on or something), and pin it and sew it to the bottom layer of the second tier. Confused? Hopefully this will help:
Now you've got a nearly constructed skirt - all you have to do is sew up the side of all three tiers, and then pin the top gathered edge to the foundation skirt you made, right under the waistband. Sew that all up, and then bam! Put on your petticoat and twirl around!