I wouldn't attach elastic to the skirt, the Lycra will act as the elastic for you. I doesn't really matter if you add the skirt into the seam or sew it on after, if I have a seam, then I would use it, but if not, I prefer to mark where I want the skirt to be and sew on, that way if there is a change of mind, or the child grows etc, you can move/replace it much easier. Just remember that if your skirt fabric has no stretch, you will need to stretch out the Lycra as you sew.