Broan has a selection of roof and gable fans that come with templates.
Here's an installation manual off their website for one of their roof
fans:
http://www.broan.com/ImageLibrary/br...s/99043055.pdf

You do not need to reuse any shingles, but you do have to cut some out
and loosen some up and remove some nails in order to slide the fan
flashing underneath. This is done by lifting the edge of the roof
( which if the roof is old you might break the shingle) so you can
expose the nail, then you remove the nail with a small crowbar or
similiar tool.