Leave em alone!
I fully agree with our friend Omaha. Unless you are over-run with them, or you have an especially aggressive snapper, I'd recommend enjoying their presence.
They've been a part of my life since I was born. We lived on the edge of a river/muskeg when I was born. One of my uncles told me, many times, that he chased away a "washtub-sized" snapper that was scratching at one of the basement windows when my parents brought me home from the hospital.
My parents allowed me to have small ones as "guests." We allowed our kids to do the same, for up to about a week -- then they went back to nature. I've toyed with bigger ones. I've accidentally caught them while fishing. I've saved many. Many have been dinner.
In general, they do far more good than harm.
If you must take one, be respectful.
In any case, be careful. I'll let someone else describe how to best "dispatch" one. But, if you do it my way, be aware that an amputated head can be dangerous for quite some time after removal from the body.
If you plan to have one for multiple dinners, it is best to let it live in a tub of regularly replaced water for several days before butchering.
Ken