That is a little hard to describe in print without being wordy. Essentially, you engrave an outline around the area where the plate will go, then using a flat engraver, remove the metal which will be replaced by the silver plate. Then, using a point, onglette or similar shape engraver around the edges, you undercut the metal, forming a sort of dovetail around the edge. The silver plate, which should be just a bit oversized is very slightly shaped so it will fit in the hole. Then, the silver is hammered into place. Being very soft, it will form itself into the undercuts around the edges, which will hold it in place and then filed smooth. I am sure someone who is better at describing the process than I am will chip in. It isn't that hard but you probably need fairly decent engraving skills to pull it off. I practiced on a few pieces of scrap barrel before I felt confident enough to do it.
