Stick the wooden spike into the pumpkin (without piercing the base!).
Thread the body onto the spike. Drape a flat area of Fimo Violet, pleating the dress. Make a colombin in Fimo Violet for the arm, glue it to the dress, marking the elbow, and add the little hand in Fimo sable to the end of the corolla-shaped arm colombin. If the hand is to hold a wooden broom, add it before baking. Make the collar-arm in Fimo Noir, which should be pleated and hide the upper arm. Tiny gold metallic petals, arranged in a trio, decorate the shawl.
And why not add a bow tie?
Important: think of the fastener! Unless your witch is in a sitting position, you'll need to think of a fastening before baking: a folded wire pricked in the back and embedded in the aluminum ball works well (the fimo witch, once baked, is very light).