You but that bullet just below the head and dead center of the neck and it's lights out 100 % of the time. DRT! I'll only take that shot when necessary though.
How and when do you know it is "necessary"?
I assume you're being a smarty-pants by asking this question...whether you're a prick or genuinely interested, the answer is simple...the "how" and "when" comes with experience and how bad you want the animal in question.