You are more likely to find an answer by posting this or searching in the flock management forum. I know it won't be easy to do this but if you know it has to be done then there is no other option but to get it done quickly. Some snap necks, some drown them, some cut the neck with sharp knife or scissors. Good luck.
I like to call my brother to come over and do it....(the one was one of his chicks anyways) I ran inside but then peeked out the window. (I know--morbid)
It wasnt that bad, although he broke its neck and I didnt see him out there wielding my kitchen scissors. I dont think that Id be able to use them again after that....<shiver>.
Ive learned breaking necks is a good skill to have. I have had to kill 2 birds when the dogs got them, one was my favorite black star. It wasnt as bad as I thought, and it was better to see her relaxed and out of her misery. Doing a chick is a little harder tho. I hope its fast for you. Dont feel bad. I believe in Chicken Heaven!