Moving main vent pipe at the roof liine

I need to move my main vent pipe a few feet from it's current position through the roof.

The PVC piping part of this is not a problem for me. The problem is, what are the steps to take when cutting through the roof and shingles?

I don't want to cut away any shingles (or make a hole in them) that should not be there.

Thanks

The hole winds up being a little oval, because of the roof angle.

The shingles are cut to fit around the roof jack.
I tend to cut my hole based on how the roof jack is going to fit,
That way I don't have to cut twice.
In most cases, that means cutting more on the upper side of the hole.