Answer:
i typically pop my knees unintentionally when i walk. it just happens. i dont think i could do it on purpose. i can sometimes feel it about to happen right before it does, and sometimes it just completely takes me by surprise.
now, my ankles and shoulders i can pop intentionally. to pop your shoulders, bend your elbows and raise your fist to around your shoulder, then move your elbow from front to back fairly fast (some people need to rotate their elbows in a circular motion, just depends on the person/joints). this will pop your shoulder socket, and makes you feel more loose in the arms. now to pop your ankles, simply bring your toes up as much as you can and then move your feet in a clock-wise motion until it pops. if clockwise does not work then try counter-clockwise.
everyone is different, so what works for me may not work for you...