Unfortunately the advice stopped there. Nobody ever said what to do instead. There are fads in English, probably in most languages.
People still say that one.
And somebody, trying to avoid ending sentences with a preposition, and probably not knowing what to do instead just dropped it, leaving the sentence incomplete in many cases. Now that has become a fad too. It's one that particularly annoys me and in a lot of cases it comes out sounding pretty silly too. So lets look at some examples. That will be followed by a comment or two about the last, incorrect form of the sentence.