Sew together and hem one skirt. This can be made out of your circle skirt pattern or a a-line skirt. Put the elastic in and make sure it hits just below your waist, or whereever you want your skirt's waistband to rest.
Take your netting and cut it at least in half. Tulle comes in really big widths. I ended up cutting my netting into 1/4 width lengths, which gave me 36 yards of tulle for nine yards cost. Cutting it depends on what you want-- Cut it to be at least the length of your skirt. You can cut it to be just that length, or cut it in half and hold the tulle over for extra poof.
Take the white yarn and wrap it around your skirt close, but not on the waistband. Make sure its long enough to touch all of the fabric, including folds/ruffles. After you know what length you need, cut the yarn and gather the tulling on it. You can do wide stitches and pull the tulle at the end, but it's easier to just push bunches of tulle on the needle and pull it onto the yarn when the needle is full.
After you have the length of yarn covered with tulle, pin it to the skirt. Depending on how poofy you want it, you'll want to start with the bottom row of tulle or with the top. The top tier of tulle is sewn to the skirt one inch below the elastic waistband. The second tier should go about a third down from the top so the bottom of the second tier matches up with the bottom of the top tier.
Pin (Carefully) and sew the yarn down. It helps to tie the yarn ends together so they stay nice and neat. After both tiers are sewed down, hem your other skirt piece (This piece should be the circle-skirt pattern, as full as possible.) Fold over the waistband circle down once and sew it down.
This is the point where you want to add the lace to your skirts hem. Trim any extra bunches of tulle from the waistband and make sure the tulle is even on the bottom.
Pin the waistband-less skirt on top of your tulle, making sure the seam matches up with the bottom of the elastic waistband and hides all the tulle. You can do without the top skirt if you have softer tulle, but the extra layer really helps with containing the tulle.
Sew the top skirt to the rest of the petticoat and you're done! It is REALLY important to iron everything well before you sew it, because it is nearly impossible to deal with when you have everything sewn together. You can wash petticoats, on a gentle cold cycle. Hang it up to dry, or the dryer will shrink the tulle and it will never sit right again.
