On a 4/3 rib sheet laid out with the left side of the sheet covering the right side of the previous sheet(guessing you have 4) you want to place them in the 2nd rib from the rib covering the previous sheet.
Blue=High rib with highest screw in it
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This prevents snow and ice pushing the next sheet covering the 4th rib up between the screws and causing water to leak onto the insulation.
Be smart about it and use some scaffolding or something along those lines under where your working in case you slip.