How do you install metal roofing? Where do you put the screws????

I am installing a metal roof and heard of many different ways of anchoring the R-panel to my existing roof. I have been told to install the screws on the low side of the R-Panel with self sealling screws. I have also been told to install the screws on the upper part of the ridge. The second option made some sense because why would you make a hole where all rain and snow would be running down. But the first option makes sense because it is a solid anchor, no gap, between the R-Panel and the wood underneath. PLEASE help me ,
Thank You!!!!!!!!!!!