I made this homemade decapitated woman costume all out of paper mache, I formed the upper body and arms out of wire and tape then paper mache'd it and painted it, for the cut neck to look more realistic I used the expandable foam in a can.
The dress is a long formal from a thrift store that is large enough around for the fake body, the backpack and my own body. I cut a slit in the dress where my face/head could fit through and the fake arms hold my head, my real hands were under the dress, (it was great to wear to work since I had an excuse I couldn't work). The arms are just duct taped to the shoulder area on the upper body part so that they are semi-movable and I even painted the finger nails on the fake hands. I added a necklace around the neckline.
What is keeping the upper body in place is a child's backpack from the dollar store and I cut a 1 inch dowel rod in half and stuffed the backpack with some newspaper for cushion and then to hold the dowel rods in place I used the expandable foam all around inside the backpack making sure that the foam was attached to the inside of the backpack and the dowel rods.
It took me 3 weeks to complete it. I added a lot of blood to the dress and my face. When the homemade decapitated woman costume is on, the dress will be raised and it will make the dress only come to your knees, you could add to the dress to make it reach the floor, but I didn't. When completed it will rest right above your real head. I did get first place for the scariest at work.