How can I "pop" a dislocated shoulder back into it's socket?

You can't. Unless it has come out of joint many times, it requires a trained physician to relocate a dislocated shoulder.
Don't try. This is best done by a medical professional under sedation. However, in an emergency situation such as if it occurs when you are alone in a remote location, you can clasp your hands together in front of you and around your bent knee (with hip also flexed). Next slowly straighten or extend your hip placing a pulling or distraction force on your shoulder.