Easiest ever Mickey Mouse cake topper! :) 

4.

Cut them both in half as shown in the photo. The blade should be clean and dry. That way you will be able to make clean cuts. 

If the paste is still quite soft/ squishy, leave it to dry for more time before cutting. If you make the cuts when the paste is still soft, the body will lose its shape and you will not get a sharp edges.

5.

You want the top surface of the bottom red part be the same size and shape as the bottom surface of the top black part. 
 

6.

Check if they fit together.