How to create Spotlight searches to store as Smart Folders in macOS Sierra

Apple introduced Smart Folders years ago, but they’re often given short shrift compared to other macOS features. While recently responding to readers’ troubles for Mac 911 and researching other stories, I discovered I’d mostly given up on a powerful option for saving recurring searches.
This is partly due to Spotlight. For many years, starting with its introduction, Spotlight indexing could be spotty or crash and re-run, and searches weren’t always as comprehensive and a