The panel that is going to be put on is normally larger than need be. Mark it out to the size that you need to cover the area that you are going to repair. Trim this new panel straight using either a GOOD set of snips or preferably a set of electric shears. After trimming it to satisfaction, lay this panel over the panel you intend to replace and mark the old panel. Cut the old panel out leaving 1/2" to 1" of extra material. After cutting out the old lay the new up and make sure you still have some material to trim. Start trimming down until you are satisfied with both panels fitting with a butt joint. Another way that some do it is to purchase a flanging tool to flange the original panel and lay the new into it. It will only flange enough to make both panels flush when put together. Usually you can pick one of these flanging tools up at the local swap meets at almost any tool vendor. One side of the tool flanges while the other side pierces a hole. With this you have a hole to spot weld thru. Then finish as with your desired coatings and fillers.