This is going to sound super grusome but what I have found that works the best for me is to put the chick in a brown paper bag, put your hand in and position the chicks neck so that it is between a super sharp pair of kitchen sheers, pull your hand out and clip off the head. It has worked really well for me several times, there is no mess, you dont have to see and it is over super quick. I have also heard of people putting chicks in an air tight container with a rag that has auto starter fluid on it. I hesitate to try this myself as I am worried the fumes may be worse then just removing the head....
Quick neck snip with kitchen shears or even garden shears. Gross, but fastest and most humane.
You will get tonnes of different ideas, I am sure. Lighter fluid, drowning, cervical dislocation, methods of suffacation.... You need to do what works best for you and you feel is the most humane for the chick.
I was just reading about other people having to cull their chicks for one reason or another, and I was like "AHHH what if I have to do that?!?" I don't think I can. Good thing I have a hubby, in case of that horrible necessity!
For chicks I using the carbon dioxide method. Baking soda and vinegar in a medium pot with a steamer basket in it. I fill the bottom with vinegar (making sure it's not to high that it would get chick wet) , put the chick in a sock so he feels snuggled, add a couple of tablespoons of baking soda, put on heavy tight fitting lid and walk away for 10 min. They don't chirp after 15-30 seconds but I wait 10 min. just to be sure. Thankfully I have only had to cull very few chicks and it was always due to a defect that despite my best efforts was not fixable.
I had to do this one time. Had a chick hatch with all of its innards in a sac outside its body. The chick itself seemed happy go lucky but because there was no way it could survive..... We use a .22. it's quick, effective, and any mess is kept outside not in our home.