I'm with those above who mention a sedative for the animal before the final injection.

I've had to assist twice with dogs in the last year and a half. I told the first vet that if he could not assure me that it wouldn't go the way it had when a vet put down my cat several years ago, I would not allow him to touch the dog, that I would be taking the dog home where he would die in his own time and in his own way.

He gently promised me that it would be painless, peaceful and fast. And it was.

I disagree with people who believe dogs don't have a concept of death. They do.