Go into your attic and mark the location for the vents, usually centered between trusses/rafters. Then drill a small (1/4" +/-) hole through the roof sheathing at the marked points, from the attic side, and push an old bolt through to be able to easily find each from the top side. Easiest to make a cardboard template of your vents' outside protrusion dimension, plus a half-inch. From the roof side, set the template at each hole, and then adjust up or down to maximize the upper (overlapping the vent's flashing) course of shingles. Scratch the outline of the template on the shingles, then remove them (and a few nails you'll encounter) with a utility knife, running the blade along a short piece of steel or heavy plastic to guide the cuts. Then use a Sawzall (drilling a few adjacent starter holes at the cut line) to cut the sheathing. Prepare yourself for a whoosh of very hot air and sawdust.