Divide the area of your attic by 150 to find your NVFA and determine how many of each type of vent you’ll need to install (compare the area you need to cover with the area each vent covers, as stated on their packaging).
 To install the vent,


Remove all cap shingles.
Starting 6 inches from the end of the roof, cut through any shingles about 1.5 inches from the ridge, and use a power saw with a blade depth of about .75 inches to cut through the wood sheathing and create an opening.
Install the ridge vent according to the type of vent you’ve purchased: for a shingle-over ridge vent, pre fasten the first section and add others until you’ve covered the entire opening, and for metal ridge vents align the vent over the opening and fasten with 2 inch roof nails.
Metal ridge vents will require straps over the joints that will also need to be fastened with roofing nails.
Shingle-over ridge vents get a layer of shingles so the look of the roof is less interrupted. Cut the cap shingle to size and reattach.
 Without both sets of vents, air will not circulate through the attic and you’ll need another method of keeping it cool. However, for an easy, energy-efficient method, install soffit vents:


Determine the placement of soffit vents that will be free of objects and other obstructions. Remove the insulation that creates a barrier between you and the site of installation.
Use a power saw to cut an opening for each of the soffit vents. The opening should be just slightly smaller than the vents themselves. Discard the removed pieces of wall.
Place the vents and use screws to attach them to the eaves, and caulk the edges to ensure a tight fit with no air leakage.
Reapply the insulation, though if the insulation prevents airflow in through the soffit vents and out through the ridge vent, insert baffles.
Measure your net free ventilating area (NFVA): Attics need a certain amount of ventilation based on their square footage.
Install the ridge vent: The ridge vent will create a space for heat to escape the attic, which is why these vents are at the peak of the roof.
Install the soffit vents: Soffit vents are located lower in the eaves of the house and draw in cooler air (forcing the hotter air out through the ridge vent).
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