1. In the Arrange window, create a Track for each Instrument.
Please note that you can prepare a piano (split) staff from a single Track, there’s no need to create one Track for the bass clef and one for the treble clef.
This name can later be used in the Score if you like.
3. Record into the Tracks or create empty Parts on all Tracks.
You can make very long Parts that cover the entire piece, or you can start out with shorter Parts to begin with. If you choose the latter option, you can always go back and add new Parts or copy existing Parts later.