(Photos 1 and 3), plugging major air leaks into the attic (Photo 2) and correcting any other of the common causes of poor attic venting.
Even if you aren’t having problems, bring your attic venting up to code when (1) you install new shingles and (2) you add attic insulation.
To install the ridge vent, first pop a chalk line down from your ridge line that’s equal to half the width of the ridge vent.
You can buy special ridge cap shingles for laminated shingles like we show here, or cut your own ridge cap from three-tab shingles (the shingle packaging will have directions).