you will need to cut an opining on each side of the ridge board. most manufacturers recommend removing 3/4-1 inch of sheeting on each side to allow the attic air to escape to the ridge vent. adjust your circular saw to just cut thru the sheathing without cutting the rafters. use a chalk line to snap a guideline on the sheathing where the cut is to be made.

the ridgevent packaging should contain instructions for the preparation and installation of the ridge vents. also make sure that you have adequate soffet vents. square inches of soffet equal to square inches of ridge vent opening in the sheathing

JMHO

Carl