One way is to obtain a suitable barrel blank. Then turn it to the contour you wish. Thread and fit it to the action with the correct cylinder gap. Correctly cone the inside of the barrel where the bullet enters. Brownells sell this tool. Install a front sight if required. Attach what ever else you want. (The ejector rod housing for a single action ?) In double actions you may want to add an ejector housing and "lock"
Another way is to obtain a factory barrel if possible and fit it.