Plan and mark the location of your roof vents from the attic so that you can avoid placing vents over the rafters or the framing. Make a template 1 inch smaller than the vent using a small piece of cardboard box, then place it on the ceiling where the first vent will go. Drive nails through the sheathing and shingles on each corner of the cardboard box to mark the vent location so that you can see them from the outside.
Outline where you will be cutting the vent opening with a piece of chalk on the rooftop. Cut the shingles with a utility knife making the cutout area about ½ inch larger than your outline. Peel off the shingles. Drill a hole on the four corners of the outline. Place the blade of your reciprocating saw in one of the holes, then keep cutting until you cut out the vent outline you made.
Separate the adhesive from the shingles with a pry bar to keep them separate while installing the vent. Remove nails that would prevent the new vent from proper installation. Slide the vent over the hole, and nail in some roofing nails over the lower edges. Secure the vent from leaks by applying roofing cement where the shingles connects with the vent.