How you make a petticoat depends a lot on how much "poof" you want, if you prefer a "princess" shape, or something with a gathered waist, and how heavy the fabric is that it's going to be holding up.
For a basic simple petticoat, you'll want netting, some broadcloth to line it, and elastic for the waist. And a good iron for the broadcloth.
In terms of measurements, you'll want the petticoat to be a few inches shorter than your skirt so it doesn't peek out, and it should sit a little below your waist to keep things from looking too bulky. As the average Lolita skirt is 20-21in long, that would be around 17in.
I suggest something that's about 3x your waist measurements, so 1 yard is just a generalization if you have a 20-something inch waist.
Cut 4 rectangles of broadcloth-- 36x18in. (Two for the outer lining, two for the inner lining.)
Cut 2 rectangles of broadcloth-- 36x3in.
Cut 4 rectangles of netting-- 36x11in. (Two for each layer.)
Cut 8 rectangles of netting-- 72x7.5 (or 16 rectangles of netting-- 36x7.5in. Also, though it looks better, netting does not need to be hemmed. Make it 8.5in if you plan on hemming it)
Take the first two pieces of broadcloth for the lining and, wrong sides facing out, sew down the shorter ends. (optional, but suggested: serge both seams). Press the seams and hem the bottom (fold up 1/2in, press, fold in again, press, sew). Repeat with other two pieces of broadcloth. Set both layers of lining aside.
Take the first two rectangles of 36x11in netting and sew together shorter ends. Repeat with other two 36x11in pieces. Set both layers aside.
Take the first two rectangles of 72x7.5in netting and sew together shorter ends. Repeat with other 3 pairs. (Or, if opting for 36x7.5, sew two together, then another two, then sew those pieces together so you have a big loop of 4 pieces sewn together, and repeat with the rest until you have 4 loops).
Taking the first two 7.5in wide loops of netting and place one inside the other. Use a basting stitch along the top. Gather it onto the bottom of the first of your 11in wide loops of netting and sew. Do the same with the other two 7.5in wide loops and the other 11in wide loops.
Place your first finished layer of netting inside the other. Take your two layers of lining and put one outside the netting, and one inside, with seams facing the netting (So you'll want the finished side of the underlayer against your legs when you're wearing it, and the finished side of the outerlayer showing when it's on). Use a basting stitch along the top of the 4 layers to keep them in place.
For the waistband, you'll take your two 36x3in pieces of broadcloth and sew them together on ONE of the shorter sides, but not both, so it's a strip, not a loop. Fold it in half so that it's 36x1.5in and press. Then, the tricky part is going to be taking the raw edges and folding them in .5in and pressing them so it's even and finished. The end result should be 1in wide. You'll take this and tuck the basted end of the 4 layers of fabric inside, pinning it together, and then sew all the way around, just a little bit inside the bottom edge of the waistband. This makes the casing for the elastic. Since you'll be adding elastic, you'll want to overlap the two ends of the waistband when you're sewing it on, folding the top end under, but wait to sew it shut.
You'll want your piece of elastic to be a little bit smaller than your actual measurement, so it won't slip down, but won't be too tight. Generally, cut a piece either one inch smaller than your measurement, or exactly at your measurement. When feeding it through the casing/waistband, I suggest pinning one end of the elastic to one end of the casing, so it doesn't get stuck inside.
Feed the elastic through the casing, overlapping the elastic by 1in and stitching the ends together. Then stitch the casing/waistband shut. Viola! Petticoat.
...I really hope that was clear enough. oo;
