There are three main skirt pieces. The circle skirt, which depending on the dolls scale can either be the big circle which is used for most fashion dolls or the small circle which is used for very small dolls. The straight skirt is the rectangle piece. For dolls taller or wider than a Barbie you may sometimes use two of these to make this skirt. The flared skirt with train is a two piece skirt, the front piece is like a triangle with the top cut off and the back piece has a circular train at the bottom. It can either be worn at the waist or up high as an empire waist. There are also the combo skirts. for a mermaid skirt, make a straight skirt and add a circle skirt around the bottom.
You can use multiple flared skirt pieces to make the above skirt combinations.
You can use a small circle on top of a circle skirt for an overskirt. Also you can cut a slit in the front of a large circle or small circle over skirt and fold back the edges to make the overskirt third over from the left in the picture above. The straight skirt overskirt is made like a circle skirt but with the straight skirt piece.