1.you are going to pop some shingles. before you put the vent skirt on the top shingles, make sure you tar the nail heads (as shown in the video, but I felt he did not do all of them), any nail heads that you expose, even if it is hard to see or hard to reach.
2. tar the underside of each shingle you lift, all sides, not just the single line per piece as in the video.
3. I would put a heavy line of tar on all sides of the vent skirt on the underside.
4. when the vent is in place, tar around the vent skirt top side, any joints with the shingles, whether the shingles cover the skirt or the skirt cover the shingles.
I can not over emphasize how important it is to properly seal vulnerabilities. Properly flashing is the most important job of roofing, but a good percentage of roofers don't know how to do it.
