If live spell checking is enabled and you write an unknown word the spell checker will underline it in red:
Right clicking the underlined word will bring up a menu with several choices:
1.Click the word you want instead.
2.Ignore.
3.Add to spell check dictionary (if legit).
4.Bring up the spell check dialog.
5.Activate the auto-correct dialog and make sure you'll never have to rectify this error again.
Live spell checking may be disabled in the spell check dialog.
See also:
Custom dictionary contents
Dictionaries