Use a pencil to poke a hole right through the centre of the Styrofoam block. The hole should be slightly larger than the diameter of the drinking straw.

Carefully make several long cuts in one end of the drinking straw and then insert the straw into the Styrofoam block.

Push the centre of the wheel between the slits in the straw. Make sure it's mounted quite securely on the end of the straw.