I will take a stab at it, people more knowledgeable than I am can correct me. fowler sights are almost always soldered on.
You mark a centerline on the top of your barrel first, I assume you have an octagon to round barrel. On a flat surface, place the top flat of your breech on the surface, place a tri-corner file in the area where you want your sight and drag the barrel backwards over the file lightly. This will leave a faint centerline mark.
This is an underlug on a fowler but the process is the same, you can see the faint centerline. You can also see how well the lug conforms to the barrel curvature.
You need to match the underside curvature of your sight to the barrel. To do this, wrap a piece of 220 grit sand paper around the top of the barrel where your sight will be and slide the sight back and forth on the sand paper until you have a perfect fit sight to barrel.
At this point you line the sight up with the centerline on the barrel and solder it on, tall end facing the breech. I like solder paste, quick, strong and easy.
My fowler sight is centered 2 1/2" back from the muzzle. The front sight position is not changed for old eyes, the rear sight is. I looked a bunch of fowlers in the Flintlock Fowler book and found them all placed like mine or slightly further back.
Here is the sight on my fowler, I filed a lot of the base away so it wouldn't be a lump out at the end of the barrel. My fowler was hitting very low with round balls (I have a rear sight) so I filed the sight height down quite a bit to get on target. The sight is the only thing on my fowler that is brass, I have an iron sight but haven't taken the time to install it yet.