I am sure there are several ways to do it. Once the material has been removed I take a small drill bit, 1/32, and drill shallow holes at an angle around the bottom of the cavity, right at the edge, just 3 or 4, they don't have to be very deep, take a pencil and color the cavity and holes.
Do not over heat the pewter, it doesn't take much to melt it and once it does you do not want to over heat it. As a test of the temp you can try heating the pewter,once it is melted touch a small piece of wood in it if it flames or blackens it is to hot if it just browns up a little you are ok. Once you do it a few times you will know how long you have to keep the torch on the pewter and how much real working time you have. I would suggest you try it on a piece of scrap before going on to the real thing. Hope this helps.
Tim C.
PS: If you are talking about a bolster or an inlay that encircles the handle that is another story.