what is the final step in correctly matching ammunition to a firearm?

Direct answer: The final step is to confirm that the firearm and the ammunition are matched by comparing the barrel/gauge markings to the ammunition markings and ensuring they correspond before loading or firing. In practice, this means you verify that the caliber or gauge, and any length or magnum designation shown on the barrel or slide, matches the headstamp or box labeling on the ammunition you plan to use, and that those details align with the specific cartridge or shotshell you have. Only after these markings are in agreement should the ammunition be loaded and the firearm used.