Best punch to use against someone in an opposite stance (e.g. orthodox vs southpaw) is the cross.

If you want to use the rear hook, no matter then stance, I would use it when your opponent has a high guard (he may leave the ear area open) where you can sneak it in around the back. Like what Mayweather did to Cotto time and time again. He whips it in (taking speed & accuracy over power) but doesn't follow through.

Overhand is a looping punch that comes from above in a semi-circular motion. You mentioned that you're a southpaw so presuming you're fighting against orthodox, use it when your lead foot is on the outside.

1) As your opponent is coming in with a punch, dip your head to your right as you throw.
2) He'll end up missing (may drop his guard) and if he threw a cross or jab, he'll falling into your punch.