My daughter helped by painting purple stripes on the bottoms of an old pair of her jeans.
I cut the profile of a witch's face (actually from the Wizard of Oz) out of plywood and painted it green.
I stapled on an old witches hat (my daughter once used as part of a costume) and I cut another piece of plywood for the upper body and attached it to the head. Then I put an old shirt on the witch and stuffed that and the pants with old newspapers and grocery bags. In order to get her to stay on the bike, I screwed the body piece of wood to another piece that went across the shoulders and screwed on to my sign post. Then I added old shoes and gloves as well as a cape. These were all things I had already, so this didn't cost me anything to make.