OK, it is too complicated of a job for me, and most plumbers ..... so how DO a well trained plumber do it? Please use layman terms. Lots of words you guys throw around is beyond me, but I am willing to learn.
For starters I never use an open flame when repairing a lead waste line as that is foolhardy UNLESS you know how to use the feather of the flame not to burn a bigger hole in the lead
I clean the lead with shave hooks or a wire brush on a drill wearing face mask and goggles
I then take a ladle of molten solder normally 40 - 60 and place a cloth under the pipe /fitting to be repaired and getting a wiping cloth start to pour the solder and as it solidifies wipe the joint
The only reason I still do this type of repair is some of the homes I work in are over 100 years old and I do not want to destroy the marble or inlaid wood floors