There you go, you are catching on pretty quick 
Keep in mind where your new regulator gets it's input power from. If the nano become powered from the USB only then it won't be able to input power to your new regulator (USB +5vdc power maxes out a 500ma and regulator input voltage must be higher then it's output voltage) Also your external power must be able to supply the combined total current used by the Nano's regulator and your new regulator.
Lefty