You need to determine the OD of the two tubes your stem will unite. Then you get tubes that will have an ID the same as the OD you will go over. The .058 wall is normally the correct size for a pinch fit, that can be easily positioned. Pinch bolts are sold by Nova etc... and may fit out of the box or require a little work to achieve the correct fit. You braze the bolt on the tube and then slit both subsequently. .035 should be fine for the piece in the middle.
Paternek has a tape on brazing racks and stems and it is a pretty good deal. He recommends doing the joints on a mill because he says that the smallest error will be apparent in the bars, he is fine with cutting other joints on frames by hand. I have a mill so I really don't know if there is any particular reason to rule out hand work. I kinda doubt it as long as you have a means to accurately measure it, I don't see a problem.