(For those non pilots, the pitot mast holds the Pitot Tube which hangs out in the wind and measures the relative airspeed of the airplane. Pretty cool and necessary to fly. Its a french word and pronounced PeeToe.. ;-)
You start by using the template just outside of the last access hole. On the 7 you need to work past a rib, and make sure that you center the rivet holes behind the template. This puts the inside edge of the pitot mount about 3/4 inch to the outside of the rib.
I used a combination of step drill and grinder to carefully cut the skin. Checking the mast all the time to make sure of a good fit.
Once the mast fit through the hole, I match drilled the holes through the spar and to the back of the tube from the template.
Note that only the rear two holes get countersunk because the three in the front fit behind the spar and it is the spar that will be countersunk.