Hold the Turkey by the legs with it's back towards you. Lower it's head to the floor so it puts it's neck paralell to the floor. Place a stout broom handle or similar over it's neck, stand with a foot on the handle on each side of it's head, then pull up sharply on the legs.
Do this last bit quickly, if you hesitate whilst standing on the handle you are just choking it!
Alternatively use an air gun to shoot it in the head.
It's important to have it's back to you as it will flap violently once you break it's neck/shoot it and those wings can hurt if the breast is towards you.
Good luck
Dave