Don't press your feet firmly on the broomstick until you're ready to pull. You must first gently find where your feet must go. This way the bird doesn't get stressed.
You have to remind yourself that the chicken is a farm animal and the most merciful thing you can do to it is put it down. I talk to my birds before I do "the deed" because I want them to have a peaceful death. Don't feel guilty. Sometimes we have to be brave and do difficult things.
Put the broomstick across his neck right below his head. Place a foot on the broomstick on either side of his head. Just pull vigorously till you hear or feel a cracking. If you're worried about him not dying, I think it is always safer to pull harder. Don't stop pulling once you start. You...