Place barrel on flat table with the top of the octagon down. Place triangular file under round section of barrel where you want the sight affixed. Lightly move the file fore/aft to make a light scribe mark to mark the centerline. One can also add marker or other to that area to help see the faint line resulting from moving the file under the round section of barrel.
Then there’s plenty of tips & pictures on here about using a short piece of hacksaw blade or similar as a ‘spring’ to hold it in place whilst soldering it.
