Real marbling takes quite a bit of effort and requires oil paints, tray, water and mess! Here is an easier version for younger children which still produces a very pretty effect.
You will need:
A paper plate
Put a blob of paint in each colour that you want to use onto the plate.
Use the fork to draw swirls.
Move the plate around a little so that the paint runs, if you like.
When the paint has dried you can cut the edge off your paper plate if you want.