During sex, either anal or doggy style, shit on your fist and deliver a violent jab to the face or back of your partner's head. The punch will only count as a shrek punch in the event sloppy greenish shit is used on the fist.
"Man, I'm never going to get a true shrek punch on Linda. My shit is always too solid and brown..."
"Don't worry, you'll get lucky one of these times. Just eat more Taco Bell and White Castle."