where to mount rear spoiler on 73 duster

thanks i am a little nervous about this, but i am going to try it, do you think i have to cut the top side of trunk lid or just the underneath, its still in my back garage so i have time to figure this out, wing has not been bought yet, i live in michigan, so i cant do nothing yet, there is more snow here now than i have seen since i was a kid and when i actually enjoyed it, thanks