Instructions
Go to the Library level, which is the first level with one of Donald's parts; you reach this level after defeating The Stretching Room level. Jump into the projector to arrive at the library. Walk down toward the end of the library. Several bookcases fly through the air, but only one remains on the right side. Use the paint thinner on this bookcase to open a secret room. Enter the room and open a chest to reveal one of Donald's legs.
Open the chest behind the organ in the Ballroom level. The organ is located on the stage in the ballroom. Walk straight toward the organ. Look across the curtain toward a blue chest. Jump over this chest and climb the balcony. Fight off the villain that awaits and then capture Donald's arm.
Enter the projector at the end of the Ballroom level to enter the Mad Doctor level. You then enter a room with a large machine. Create a wall next to the large machine. Climb the wall so you can reach the top of the machine's conveyor belts. Climb across the belts and jump to the hanging ledge. Just past the ledge you will see a blue chest. Open this chest to reveal Donald's other leg.
Enter the mansion in the beginning of the Back to Bog Easy level. It is the first large building you see as Mickey starts the level. Approach the man you meet upon entering the mansion, who thanks you for fixing the organ. As a result, you receive Donald's torso, the last remaining part that was missing. Run up to Donald, who appears at the end of the hall in the mansion, and speak to him; he automatically assembles himself.
Watch the cutscene where Daisy and Donald finally get together. If you missed any steps, go back to previous levels and sections to replay them so you can pick up any pieces you missed and restore Donald correctly.