Many years back I worked for a firm doing DG, the chap who did the replacement units carried a roll of lining paper, masking tape and a black crayon. Did all the leaded stuff in situ like a brass rubbing, brought it back to the workshop, laid it on the bench under the glass and stuck the lead over the pattern. Worked a treat.
That method works well for non double glazed units. Once you have glazed units you have lead on both sides of the outer pane and you still need to create an hermetic seal - not easy.