Simply put: When the power is turned off to the switch power can not flow through the switch to turn on the light. there for: The hot wire that comes in the box and goes to the first switch would need to be in the off wire... The off wire would need a wire wire nut that also has a 3rd wire that leads to the second switch. That way whether the power is on or off to the first switch, power still runs to the second switch
By the way: Green is ground, for any unstable current to be discharged.