Instructions
Talk to Animatronic Donald in the glass case near Bog Easy Emporium to activate his quest. He'll ask you to find his missing parts.
Go to the lobby of the library area in Lonesome Manor and find the toon wall next to the entrance. Press the "Z" button to coat it with paint thinner. This will reveal an alcove that contains a blue chest. Open it to receive Donald's left leg.
Jump to the balcony near the organ pipes in the Lonesome Manor ballroom, and then walk along it to the gap. Jump over it and open the chest to find Donald's arm.
Go to the trolley tracks and paint the nearby trunk. Get on top of it, and then jump to the tracks hanging above you. Jump to the rafters to find the Mad Doctor's attic. Here you'll find a blue chest. Open it to receive Donald's right leg.
Go to the Lonesome Manor ballroom and talk to the pipe organ to activate the Play the Pipe Organ quest. Play eight keys in a row as the music notes appear above them. The ghosts will then give you Donald's torso. With both Donald and Daisy fully assembled, they will reunite.