Cathy,
Welcome to the forum!
I am not familiar with the 'John Smith' brand roof vent. The most important thing with installing anything on a roof is to 'step' the shingles/flashing or flanges from the bottom to the top.
Here are some links that may help you, although not specific to your vent they are for reference to the general way to overlap the materials...
http://www.homestore.com/HomeGarden/...?poe=homestore
http://eartheasy.com/live_roof_vent.htm
I would recommend you drop be your favorite local hardware store and tell them what size pipe you have (possibly 4") ,the pitch (slope) of your roof and the type of roofing material you have (asphalt, slate, clay). With this info they should be able to fix you up with the proper materials.
If you have a specific question please let me know.
Good luck, Jim
'Just a handyman trying to help'