No direct experience with this.
Guess you thread the breech-plug into the brass.
In brass one uses coarse threads, never fine thread.
Important that the liner stay in place, not slide forward. In some of the not-so-great cartridge conversions of Sharps .52 cal percussion carbines to .50-70, the liners were known to come out. I had one of those conversions in about new condition, the changes made in the latch spring were horrible, shop that did them took no care at all, if they were the same ones that put in the liners no wonder they came out. Early .50-70 trapdoor Springfields did successfully use .58 cal Civil War barrels relined to .50-70. Trapdoor guys might know how that was done.
I don't know the best, surest way to hold the liner in. Be nice if someone with such experience would comment.
My guess would be soft solder. Epoxy tends to be poisined by copper, not reliable holding copper alloys. Yeah, I used to mix my epoxy on a brass plate & it stuck like $#*!. I recall that in Ancient Times if one needed to stick something to a piece of brass for enginering purposes one did not use epoxy. This was in the 1960's. Maybe there is a current epoxy for which this is not true. But do not take it for granted.
Didn't know these liners existed. Yeah, I like the looks of brass barrels & I am still capable of cleaning my brass. Hard wax, such as Johnston's Paste Wax, or best, Reniassance Wax, may help some. Brass is sensitive to corrosion from the sulfur in black powder so the lining is worth considering, if you want a "brass" barrel. Also you get a reasonably known steel used for rifle barrels, not whatever yellow metal one chooses to call brass.