I cut out two pieces for each side, and one piece for each of the long middle parts.
Glued them with a glue gun.
I then began putting pieces to add detail and a piece to make the keyboard 3D.
Added some cereal box cardboard on in strips to cover up the corrugations and add detail.
Printed off a keyboard from a Google images search, and trimmed it down to fit the slot.
I decopaged the keyboard on with some glossy mod podge to give the keys a shine, and painted the rest of the piano with several shades of brown to have the appearance of wood.