Hi Pat: How to install really depends on which type you got. There are internal and external baffled vents. The external baffle is the one you should install, particularly if you are in a snow area, as they have less of a tendency to be covered with snow.
The first thing to do is go up on your roof and remove the existing ridge shingles. If there is any black roofing felt (treated paper) or the top edges of the first course (actually, the final course) of shingles, use a roofing knife and cut then down to about 3" from the ridge line, to expose the underlying wood of the roof deck. Then take your power saw, set the depth of the cut to whatever the thickness of the roof deck is (1/2" if its a plywood deck and 3/4" if the deck is made of 1X6 pine boards), and cut 2" off the edge of the roof deck at the apex. That is, measure down 2" from the ridge line from one edge of the roof to the other for both sides of the roof, snap chaulk lines, and then rip these 2" wide pieces with your power saw, You now should have a see-down opening at the top of your ridge. Now, put your ridge vent over this opening and secure down using HOT DIPPED GALVANIZED DRYWALL SCREWS........DON'T USE THE ALUMINUM NAILS THAT COME WITH IT....THEY WILL PULL OUT OVER TIME. I use the rubber washers used for the old faucets to seal each screw. Be sure to use the rubber end-caps that you buy to seal the ends of and between the 10 foot sections of the ridge vents. I seal using silicone caulk.
Its an easy installation. Take your time. Oh, and don't try to short cut the removal of any shingles under the old ridge shingles by cutting them with your power saw...won't work...it'll just dull the heck out of your new blade!