The Above Knee-Highs I am working on right now has a decorative edge using Lion Brands “Ruffle.” This is a similar technique I used for the Lost & Found – a knitted skirt attached to a Lacoste shirt seen in Knit.1 The Sports issue 2007.
Here it goes: First you work1 round of St st using main color yarn. Then you switch to Ruffles and work Reverse St st (purl on right side). At the beginning of the first round using Ruffles, you fold the yarn about 3”/7.5mm back to make double thickness, insert needle into fold, going through both thicknesses at 1/8”/ 3mm from top edge. Continue going through the double thickness until you run out of the "fold" (about 2 - 3 sts), then continue to purl using only single thickness.
In this fashion work for 2 rounds to last 3 sts of the 2nd round. Then cut the yarn about 3”/7.5mm beyond the end of rnd, so you can fold it back to make double thickness, purl 2 sts trough the double thickness and purl last stitch through the fold. Then you switch to your main yarn and start working St st.
If you need further help, please post in comments.