This witch may have failed driver's ed., but she's still got a few tricks up her sleeve -- namely, the ability to provoke hysterics in every trick-or-treater who sees her.
First, choose a crash site for your witch -- a tree with a trunk that's sturdy, yet not huge in circumference. Ideally, the tree should also be highly visible to passersby.
Measure the sweatpants' legs and cut two of the boards to a few inches longer than the pants. Slide the wooden legs into the sweatpants and -- with an assistant's help -- nail a leg onto each side of the tree 3 or 4 feet above the ground and parallel to it. For the best illusion, you'll want to have at least a few inches of the legs extending toward the witch's back (so that she appears to have hips).
Slide the boots onto the ends of the legs.
Secure the broom in place -- roughly parallel to the ground -- by nailing it to the tree and one of the wooden legs.
Pull the trash bag drawstring tight, then nail the bag in place so it hangs down as a cape. Secure the cape at the bottom as well, so it doesn't blow around.
Lastly, nail the wig to the tree, cover it with the hat, then nail the hat in place. If you don't have a wig, simply cut a bunch of thick yarn into long strands and tie them together at the top.