First step in creating a bonded sand mold is to create a pattern to be cast. In this case I decided to experiment with party balloons. I chose this material because the patter needs to be made from something that will be completely evaporated by the metal being poured into the mold. In most cases, polystyrene is used because molten metal will burn the pattern away and the cavity will be replaced with metal. This is real nasty for the environment and your health. So I encourage creative use of less harmful materials for your pattern. You do not need to think about undercuts but holes can be tricky to pack.
Step 2: Step 2: Acquiring a flask
Step two: After you have created a pattern to cast, you will need a flask. This is a box or some sort of receptacle that your pattern can fit inside with at least 2 or 3 inches of spacing between the pattern and the top, bottom and sides of your flask. I used scrap plywood that I glued and nailed together. You can use just about anything that has rigidity or wont flex or bend as you pack sand into it. Try concrete cylinder forms or a metal pale in place of building a wood box.