You need some clothesline wire & some gallon plastic bottles with the bottom cut out & part of the neck of the bottle cut off (about so the chicken's head will barely slip thru).

String the bottles upside down on the wire & secure the wire ends to some posts or whatever. Put the chickens in the bottles upside down & poke their heads thru the neck of the bottles. They will settle down immediately. Using very sharp shears or a knife, slice the heads off. They will bleed out perfectly without any mess.