I used the template on the inside to drill a hole in the center. I used the hole as my layout point for the outside.
It's kind of a weird feeling cutting into a perfectly good roof! I should also mention that up to this point I've done all of the work from a ladder. Working on a metal roof from a ladder sucks.
I cut the bottom as tight as I could with a hand held 4" grinder. I over cut a little on the right on accident. It's not more than about a 1/4 inch, but I wanted it tighter. I filled the over cut with silicon.
At this point the exterior flashing is installed but not fastened. This way I can move the flashing a tiny bit to help plumb the stove pipe. After the cathedral support box is installed, I can go back outside and finish the fasteners.
One of my challenges was installing a cathedral support box after the flashing was installed above it. You can't reach down into the box because of the flashing and you can't reach up into it because of the stove pipe. I ended up putting all of the stove pipe together and using it to support the double wall higher than I needed it. (There is a collar that gets attached to the double wall pipe that holds the pipe on the cathedral support box. The collar is inside the box.)
Here's a step stool on blocks with various shop items piled up to the proper height for the stovepipe support.
We got the pipe plumbed up and started sliding the support box up the pipe. The support box with 2 X 6's attached is heavy, so imagine my choice words when the support box would not slide over the single wall to double wall converter. I battled it for about 20 minutes and then decided take the pipes apart at the transition.
The double wall pipe is heavy and the support box is to, so it was quite the chore to hold the double wall up into the flashing while trying to fit the cathedral support box over the pipe while on a ladder with my wife helping. She was on another ladder. (If anyone takes anything away from this thread, it should be: It's easier on you and your home life if you ask a buddy to help! ;-) )
The pic below shows the dings on the reducer where I couldn't get the box over it. It also shows the dent in the double wall where we dropped it. Like I said....it's heavy, we're on ladders, and my wife was helping me. Sometimes my impatience bites me in the ass.....
(The final installation doesn't have that much double wall hanging down. I'm using one of the outside pieces to prop up the pipe with the retaining ring until I could get the box nailed off.)