My son wanted to be the headless horseman for Halloween last year so... where do I start? A trip to Goodwill yielded a trenchcoat, a scarf, and a black dress shirt. Total cost $10. Cool.
I found an old backpack with wheels and the extendable handle and had my husband put some screws into the handle to lock it in the extended position. Next I cut a piece of dowel to the length of the shoulders of the coat and duct taped it across the top of the backpack handle. I wrapped the ends in foam.
Next the shirt. I got a piece of foam and cut it to fit in the neck hole. I wrapped it in an old pink shirt and glued it in place with fabric glue. I put it on my son's head and cut out a hole for his face. Next we folded one side of the scarf over about an inch and glued with fabric glue. I threaded a piece of elastic through it and secured it under the shirts collar. It covered his face and looked like an ascot. The scarf was shear enough that he could see out just fine but you couldn't see his face at all.
He put the backpack rig on, then the shirt got draped over the rig. The shirt was large enough that his arms fit in the sleeves even with the shirt on the rig. We then just put the trench coat on top of it all and buttoned it up. He carried a plastic Jack-O-Lantern for his candy that he would occasionally balance on the neck of the costume. What a ham!! We could have put a bit of gore around the neck but we were trick-or-treating with smaller children who were already pretty freaked out by the costume.
This homemade headless man costume was one of those costumes that came together much easier than I ever expected and looked so cool. I love it when a plan comes together! Total cost was about $20.