1. Measure the length you want your petticoat and 1 1/2" extra for the waist and 2" extra for the hem.
2. Cut two lengths of fabric, one for the back and one for the front
3. Sew these lengths from the bottom edge to within 9" - 10"of the waistline edge.
4. Fold each waistline edge over 1/2", then 1" more and sew close to the bottom fold. This will create 2 channels for drawstrings. Use something that's not slippery for the drawstrings, or your ties will not stay tight. I prefer linen or wool twill tape, rather than modern twills or ribbon.
5. Fold the hem edge up 1", then another inch, and sew to create the hem.
There is another way to do the petticoat waistband: Instead of sewing casings and putting a tie thru, you can make 2 waistbands (1 front, and 1 back) that are 1" wide, and use hooks and eyes to fasten it. This works really well!Here's a link to more information about women's clothing in the 18th century