"Autocorrection" is grate! I mean autokorekshun nevere fales, write?
OK, the above simply proves that I need more help than most, and the Mac usually does fine correcting my extremely poor spelling skills. But there are times when I want to spell a word in a particular way. The "normal" way is to either leave the little red underlining. But the kind folks at Apple have designed a way to "have things our way"... sometimes.
For example, suppose you have a screen name like "xabd". First, I urge you to cease and desist, or you will be hearing from my lawyers! But with just a bit of typing in System Prefs->Keyboard/Text tab will stop the annoying red underlining. Simply click the "+" button, type in the characters you want to use (without erer notification), hit the tab key and enter exactly the same characters. Wah Lah! Your Mac now 'knows' you want that collection of characters in that particular order. Thank you very much! No third-party typing/text-insertion app required.
Just another reason to look through System Prefs to see what might be useful! What's your favorite System Pref? How have you changed settings that make using your Mac easier? Will iOS ever catch up?