Find a nice selection of round objects for tracing. On a single large sheet of watercolour paper trace these out in pencil.
Add the highlight. Select one to be the Earth...the big blue marble. Add the continents.
Using watercolour paint begin painting your marbles. Reference photos help with choosing colour combinations. Allow your colours to mingle a bit. Don't worry about staying in the lines as we will be cutting these out.
Paint your Earth marble. Lift off some clouds by blotting with a kleenex while the paint is still wet.
Let your painting dry.
Cut out your marbles and glue onto the black background paper. Overlap marbles to give appearance of depth.
Finally using crayon or pastel add lines for back wall or corner of the room.