There are two versions of the petticoat, one for Faldbúniningur and a different one for Upphlutur and Peysuföt. I elected to do the one for Faldbúningur, but intend to sew the other type in a very nice, rather heavier, linen and use the pretty linen lace on that.
One needs about 3 meters (yards) for each petticoat and they are easy to make. I use a size 40-42/10-12 US/12-14 UK clothing and am quite tall, but these measurements can easily be adapted to fit other sizes.
There are basically 3 pieces, front, back and the bottom flounce (I hope that's a word). The front is not gathered but has an elastic. The back is wide and has 4 folds, two on either side which cause it to be slightly longer in the back. The flounce is four rectangular pieces of fabric sewn together and gathered.
These instructions are probably not detailed enough for someone who hasn't sewn before, but for most they should be enough.
Start by measuring your waist. Mine is 85 cm / 33.5". Divide by 2 (42.5 cm / 17") and that is the width of the front at the waist. Add to this 5 cm/ 2" (2,5 cm / 1" on either side) and that is the width of the front piece at the bottom.
Measure the length from your waist to the floor (add height of heels, if you usually wear heels). Subtract from this about 10 cm / 4" because the petticoat is longer in the back. My length is 109 cm / 43" and I made the length of the top half 50 cm /20" and the flounce also.
The back piece is the really interesting part. It is basically a 1/4 round. It is wider than the waist, but gets pulled in by 4 folds which make it drape nicely in the back. I simply drew up a circle with a radius of half my waist measurement (42,5 cm /17"). Then I measured 50 cm /20" from there, to get it to be the same length as the front piece. The flounce is 4 pieces of 50 cm/ 20" times the width of the fabric, which should ideally be 150 cm /59". The flounce could be slightly less gathered (i.e. 3 pieces) if the fabric is thick. Also cut the waist bands, slightly longer than half the waist measurements and about 5 cm / 2" high (half that when finished). Add a seam allowance to all pieces, at least 1 cm/1/4" when you cut your fabric.
Start by marking the folds in the back piece: Mark the center back and measure 4 cm /1,6" on either side and fold to the center and let the two folds meet without overlapping. Now measure 8,5 cm / 3,3 " away from the center fold and fold 2 cm / 0,8" towards the center on both sides. There should be 6,5 cm / 2,5" between the folds. Stitch down.
Traditionally there are open slits in the sides, but one can sew in a zipper or add pockets to close them, but here I didn't do that. Hem the slits and then sew the top pieces together at the sides below the slits. Sew all flounce pieces together in a circle and then sew two gathering stitches at the top. Start to gather te flounce to fit the top. It's easiest to gather each of the four pieces to fit a quarter of the top so that the gathers are nice and even. Sew the gathers to the top, right sides together. It's also good to stitch the gathers to the top piece from the front about 2-3mm / or an 1/8" from the seam.
Sew the waistband to front and back pieces, first right sides together. On the back piece, fold the waistband over in the middle and fold in the seam and stitch in place. On the front, cut a wide elastic to be about 10 sm / 4" smaller than the waistband and pin in place on either end. Fold the waistband and the seam in and stitch in place. Make one seam along the length of the waistband over the stretched eleastic to hold it in place. Make buttonholes on either side of the waistband of the back and sew on two buttons on the front. Or use snaps. Finally add a lace to the bottom and hem the petticoat.
The petticoat for the Upphlutur and Peysuföt is identical to this except the front piece is all the way to the floor, so that it has a flat front, but the flounce is on the back. Both are slightly longer in the back than they are in front because of the folds in the back piece. I think that there is a certain charm in that, but since I'm still crocheting the bottom lace, I can't really try it on yet. The crochet is a lesson in patinence, since the hem is 6 meters / 6 yards long and the 1850's pattern I'm using is 12 rounds. But I'm almost there and then I'll have a lovely long summer skirt. After all, there is a precedent for wearing ones underclothes as the national costume, so why not.