I held the chick firmly in my left hand, and with the right, ran my fingers over her neck to calm her, speaking to her all the while so she would be somewhat calm. I placed my thumb and forefingers of my right hand around her neck and gave a swift but gentle tug, and heard a pop, and dislocated her neck, killing her instantly. For me, this was the quickest most humane way I could to help her cross over.
She was approx 10 weeks old at the time.
I have had to do other chicks since then, and have done them all the same. They pass on quickly, and don't seem to suffer at all. It seems instant from what I have seen, and having talked to my vet <who also owns chickens> he says it is very very fast and painless.