It is easy to save a word in your private spelling directory. When a word is underlined in red, right-click the mouse and select learn spelling. Your word is added to your private spell directory.
But what can you do if you save an incorrect word? It is a few more steps, but it is easy to do as well.
The first step is to make sure you are in the Finder window. Then select Shift+ Command+G. When the window opens type in ~/Library/Spelling.
Hit Go and that will take you to your actual Spelling Dictionary File.
Check over your words and delete any that need to be removed. Be sure to delete the blank spaces that are left as well.
Now you are safe from your own foibles. Until you do it again. Doesn’t hurt to check it once and a while. After all, I managed to save the incorrect spelling of my great grandson’s name. That is absolutely a no-no.