The mantua maker's seam is described in The Lady's Guide to Plain Sewing, Book II from Kannik's Korner, which is a wonderful book--along with Book I--for anyone interested in period stitches.
Cut your skirt as normal. Put your pieces right sides together, as usual. Offset the under edge slightly, especially if you're using a thicker fabric. (I actually didn't do this since my cotton voile was so light.) Then, treating the two as one roll them as if you were going to hem it. Pin the whole seam like this.
Then, just as if you were hemming, whipstitch the seam. Your stitches should be small and fairly close together. And that's it!
The sample here is the skirt on my Lost Hope dress.