First off what you are looking at in the picture is the muzzle side of the sight, and if I'm not mistakened, there is a center line engraved on the other side! First, after measureing the origenal sight, if the rifle has one, from the bottom of apreture to the bottom of the sight base. Make a mark a little high on the new sight. you need to mount the sight on the rifle, and center it between the barrels. Mark to top of the new sight's center on top, or follow the center line, if it does have one. With a 4'x5' cardboard useing the top of the back sight, and top of the front sight, shoot at tohe bottom center of the cardboard.
This will let you know if the sight is centered properly. Now, make a thin cut right down the centerline of the sight. Shoot, and then cut the sight deeper, till it is hitting at the proper elevation. At this point, it depends what sight configuration you want, for a wide-V you need to scribe a line from one top corner of the sight to the bottom of the cut you've just made, and on the other side the same way. Now you have a line to file to! After the final shape of the wide-V, then round over the top corners. The flats of each side of the V cut, should slope slightly to the front(Muzzel side)of the sight to avoid glare. Safety files with cuttin surfaces only on one side is needed for this work. Otherwise, use extreme care not to hit the rifle with the file.
I hope this doesn't sound like Greek to you, but it is hard to explain, without pictures!