First oil the area on the barrel around the rear sight blank (helps limit you getting a lead smear around the area you are working on), then place a pure lead slug in the dovetail (I use round ball or conicals from my 1851 navy). You may have to flatten the sides of the slug until it just fits into the narrow part of the dovetail:

Next start gently tapping with a hammer, tap straight down at first so the lead flows into the dovetail undercuts and holds tightly. Dont whack it, just gentle taps. Lead flows easily.

As the lead expands and gets thinner, start following the barrel contour. Note when you hit the left side the lead will flow out of the right....keep doing this and alternating taps with the hammer.

As it thins out the edge of the dovetail will actually cause the lead to break off where it is not needed, you can speed this up buy breaking it off yourself as it comes loose.

You are almost there....keep tapping and following the barrel contour. watch the lines on an octagon barrel. If you tap too hard and move the line in, simply tap the other side and the lead will flow back out and straighten things up. Dont tap the barrel, tap the lead. tap straight down too, Get the lead out! In this photo below see how super thin the lead is on the side facing us? I use my fingernail to scrape it off, then keep tapping

Here it is almost done. A few more taps to straighten things out and too sharpen up the edges as the steel dovetail edges bite into the lead and shear it off. Use you fingernail to scrape if necessary..the lead is so thin at the edge that it just scrapes away easily.

Now, you are done. You can do final tweaking by CAREFULLY, using a flat needle file, filing the lead dead flat. I put very fine sandpaper on the needle file and LIGHTLY do a few strokes, but really this is not necessary. The lead, when done will be bright and gaudy, especially on a blued gun. It will oxidize dark in a surprisingly short time. i speed it up by using a toothpick to apply cold blue...it darkens the lead immediately. Just dont get it on the barrel. In my final photo you can see where I actually got a little around the lead and onto the barrel...it looks like a little dark line on either side of the lead. It is not that big of a deal on this particular gun because it is cold blue and as I wipe down the gun with an oily rag it will eventually come off the steel (but the lead will stay oxidized).

Thats it. With time it blends in with the barrel and I think it looks sleeker than a protruding steel blank.
Have done this on my Dads 1892 back in the late 1990s and it is still good and tight as the day I put it on.