Long skirts are the most difficult conforming clothing to make. Obviously, you can't conform them to the legs the way you would a pair of pants.

One way to handle this problem by putting morphs in the skirt. (Look in the body and the hip, if there's no "skirt" body part of the dress.) You may find sit and kneel morphs that will help you fit the dress to the pose.

There may also be "body handles" - red cones that let you manipulate the skirt. You pull on them, and the skirt moves in the direction you pull. (The body handles should be made invisible before rendering. Make them transparent, and they won't render.)

The DAZ morphing fantasy dress has sit and kneel morphs, as well as a body handle. Between them, you should be able to pose the witch's skirt properly.