First, the lettering did have to be carved into the stone, although the finish would be rougher than where carving alone was used. Tiny holes would then be drilled into each letter, to anchor the lead. Individually-cut lead strips were shaped and securely hammered into the stone with a wooden mallet. For raised lettering, the lead was then carefully cut to shape with a chisel and paring knife.
The time-consuming, expensive process lost popularity over the course of the last century. Original work has had to contend not only with weathering but also with metal thieves. Examples do survive, though: this one in Nunhead Cemetery shows both the quality of the end result and, in the damaged N, the carefully-drilled holes which once anchored the missing lead. 
