How was Scrooge's room transformed when he entered it ?

Scrooge awakes when the bell strikes one, and is immediately prepared for the second Ghost's arrival. After a while, he sees a light come from the adjacent room. A strange voice tells him to enter, and when he does, he sees his room has been decked out with Christmas decorations and a feast.

There was also a huge Christmas tree that the ghost sat on while he was laughing.