Start out with a black sweat suit and 2 pairs of black tights or thick nylons. Cut the legs off the tights and stuff each leg with Fiberfil. Stuff each leg until it is as long as the sleeves of the sweatshirt. If you can sew, you can make the spider legs using black material instead of black tights.
Attach two legs on each side of the sweatshirt, spaced evenly below the sweatshirt arm. You can simply pin them in place using safety pins or you can sew them on.
Once the 'spider legs' are attached to the sweatshirt, use black thread or fishing line to tie the end of each arm together. Tie one end of the fishing line or black thread around the end of the bottom spider leg; tie the other end of the fishing line to the next leg, leaving about enough slack between the two legs so they hang freely. Finally, attach the second leg to the wrist of the person wearing the sweatshirt in the same fashion. the end effect will be that the two fake 'spider legs' will look like they are moving on their own when the person moves his or her hand.
For finishing touches you can wear a black hat, paint your face black, and/or wear black mittens or gloves!
Find more Halloween fun and costume ideas in this Halloween Index
