Clean out a garden sprayer bottle.
Pour a tablespoon or two of flour, six bottles of food coloring, and two and a half gallons of water into the garden sprayer bottle.
Mix well.
Cap the bottle.
Get a styrofoam head with a neck.
Cut the styrofoam head off at the neck.
Hollow the head out about halfway up inside.
Cut a hole through the neck piece.
Make fake skin with latex.
Cover the head and neck with latex.
Paint the top of the neck and underside of the head with red acrylic paint.
Paint the face with makeup to make it look realistic.
Attach plastic eyes into the styrofoam head.
Get a headless dummy or mannequin.
Take the garden sprayer bottle and attach it to a hose, valve and nozzle.
Attach a balloon to the end of the nozzle.
Run the pump hose through the dummy neck piece, so that the balloon sticks out of the neck. It does not need to go through the body but can be taped to the back of the dummy.
Fit the styrofoam neck onto the dummy's shoulders, so the balloon protrudes out of the top. Be sure it is secure with duct tape, if necessary.
Fill the balloon with the blood using the pump.
Fit the styrofoam head over the balloon.
Attach the styrofoam head to the neck piece. It should be fairly secure but not too secure because it needs to separate when hit for the decapitation.
Clothe the dummy with a shirt or jacket with a collar, so it helps hide the attachment point of the neck to the dummy.
Secure the dummy, so the head can be knocked off without toppling the body.
Whack the head at its neck point with an ax, machete, or other weapon.
Watch the head fall off. The balloon will burst and blood will erupt out of the neck.