There are a lot of thread on here about methods if you do a search.
I've done the cutting the head off with a sharp pair of scissors. I'd say this is the most traumatic for me, but the best for the chick. I've tried pithing them and been unsuccessful, so I won't try that until I've had a chance to practice on an already dead chick. I haven't tried neck breaking because I don't know how and don't want to do it wrong. I think the easiest on the person is using baking soda and vinegar to make CO2 and gassing the chick to death, but I think that's easier on you and harder on the chick. The one thing I've heard people do that is NOT okay is freezing them, that is a horrible slow painful death.
And when, I do it when either the chick is dying or is has something that is not treatable or causing suffering. Things I've had to cull for 2 chicks that never really were strong enough, eventually progressed to laying there unable to move, culled them. A chick that was hatching missing half its head with brains visible, culled before it finished hatching. A few others, but those stand out.