Asking Siri to spell a word is my favorite feature of Siri on Mac OS 10 and iOS 10. Some say spell check has ruined a generation of spellers. Probably so, but I couldn’t spell well before computers so I can just enjoy the new feature without guilt.
Just think, you will never have to look up the spelling of broccoli or Supercalifragilisticexpealidocious again.
How do you use it? With Siri open, just ask it how to spell a specific word. I find that it doesn’t work well on some names. For instance, when I asked it to spell Gravley, it offered web sites rather than just spelling my name. It’s my opinion that it works for names if the name is tied to someone famous, but that is just my experience.