For these panties, I chose to sew the elastic on flat, before sewing the side seams. That worked fine for another pair where I used stretch lace, but not so well for this picot lingerie elastic.

You can attach the elastic using several different methods. Consult any of the books I mentioned for more options. For these, I chose to place the wrong side of the elastic against the right side of the panty leg and zig-zag in place.

Measure your elastic to the specified length and mark the center. Pin the center of the elastic to the center of one leg. Pin each end of the elastic to the side seam of the panties. Be sure that the elastic is all turned the same way. I like for the picot to be at the bottom.

Sew on your conventional machine with a narrow zig-zag stitch, stretching the elastic to fit the panty leg as you sew.

Closely trim the excess fabric from behind the elastic. Be careful--if it feels thick, double-check to be sure you aren't cutting the elastic along with the fabric.

After the trimming is done, stitch the side seams using a zig-zag stitch or your serger. I like to use a 3-thread stitch on my serger, because it is very stretchy.

This is where I had trouble with the picot elastic. The serger kept pushing the elastic ends so that they weren't aligned properly. I found that if I basted with the conventional machine, then serged, it worked much better.