For these two methods, I used 7.62x54R Brass as the parent cartridge, but you could also use 45/70 or 8x56R Hungarian, although the processes would vary. 45/70 would not require the brass ring, but would require careful sizing in a 7.62x54R F/L die, rim diameter reduction, and neck thinning. 8x56R Hungarian would not require a brass ring either, but would require the F/L sizing, rim reduction, and possible neck thinning. My Method 1 is the easiest, but you could argue that my Method 2 is the most difficult method (with 8x56R and 45/70 in between). I wanted to try the brass ring for a challenge, and because it might come in handy making 11mm Mauser from 45/90 (0.516" base for 11mm Mauser vs. 0.500" for 45/90) - I also have some 17/32" (0.531") brass tubing.

Method 1

1 - turn the rim to 0.560". I just grip the case in my drill press and use a file as it spins.
2 - anneal the neck
3 - wrap a 3/8" strip of aluminum tape around the base to bring the diameter to about 0.508". This will centre it in the chamber when it is fireformed. Note that my chamber is about 0.510" above the rim, but I have seen specs stating both 0.500" and 0.504". Your dimensions could vary
4 - fireform with 20.0 gr of Unique, under Cream of Wheat, under packed in tissue paper
5 - trim to 2.040"
6 - neck size with a 325 WSM, 8x56R, 8x52R Lebel or some other short and fat 8mm F/L die
7 - I shot 215gr 0.329" (bore slugs to 0.327") cast bullets at about 1900 fps. Nice.

This method is easy, but leaves a bulge above the rim that is unsightly, but is "harmless". If its appearance bugs you, or you just like a challenge that will develop your skills, then go to Method 2.

L to R

1 - Parent 7.62x54R
2 - Case with rim turned down, annealed, fireformed and trimmed to length

Method 2

1 - swage the body down in a 30/06 family F/L die (8x57, 30/06, 270 Win, etc.). This will allow the ring to be applied easily and without stretching
2 - apply a 1/2" ring of thin-wall brass 1/4" - 3/8" long to the case. Chamfer the edge that will butt against the rim first, or the next step, whereby you reduce the ring diameter with a file, will inevitably also reduce the rim thickness (not good).
3 - in the drill press, reduce the rim diameter to 0.560", and the ring thickness to 0.508" at the rim, tapering down to about 0.500". Test for fit in the chamber.
4 - anneal, fireform (locks the ring in place), trim to 2.040"
5 - neck size
6 - shoot. After the first time it is shot, the "joint" between the brass ring and the case body becomes almost imperceptible.

L to R

1 - Parent 7.62x54R
2 - Body reduced in 8x57 F/L die
3 - Rim turned down, brass ring applied and reduced
4 - Case annealed, fireformed and trimmed

McMaster-Carr Supply Co can supply the tubing, and they are awesome.