You can approach this DIY two different ways. The first method involves a diffuser, which will give off a steam effect when you place it inside your finished cauldron. The alternate approach involves just using stuffing to create the illusion of a bubbling cauldron. Be sure not to use both products in the same cauldron because it'll create a fire hazard.
You can buy a small plastic cauldron at your local craft store in the Halloween decor section. To mimic the appearance of the witch's brew bubbling over, you'll need a hot glue gun so you can create a "bubbly" surface for you to paint over.