From my two days experience with one of these beasts, it seems to me that the best way to sing if you are not very good is to use the bass chords only, and forget about the piano notes. Once you get better, I would assume that you can use the piano keys to chord as well. Generally, I would assume that it is like accompanying a singer with a piano -- don't play the straight melody, go for chords or simple patterns unless the singer doesn't know the tune, and go for grace notes in the silences between phrases, otherwise stay well back.

But what do I know.

yours, Peter T.