Measure out the ingredients. Place the 1/2 Cup Cornstarch and 1 Cup Sodium Bicarbonate in the saucepan and mix together. Then add 3/4 Cups of cold tap water. Stir slowly until all the dry ingredients are wet.
Place the mixture on a medium heat. Stir for 10 minutes or until the mixture thickens. The consistency should become like a thick mashed potato.
Once the mixture is cooked enough, remove it from the saucepan and place into a bowl. Cover with a lid, plate or even a paper towel and leave the mixture to stand for 5-10mins.
Now your mixture should be ready to place into the form. Preheat the oven to 150 degrees. Meanwhile, spoon in the mixture or spread it out with a silicone spatula. Make sure to get into all the nooks and corners of your silicone form.
Now place your silicone form with the mixture, into the oven. Depending on the design of the silicone form, leave the mixture in the oven for sets of 10 minutes to check the progress of the mixture. The mixture will rise a little. (In some cases you want to avoid too much heat for too long as the mixture can start to turn brown.)
Use a toothpick or a spoon to check that the mixture has cooked and become. Once it is hard, you can remove it (you can always put it back in later on if you feel it needs to be heated some more). Leave until cool before attempting to remove the mixture from the silicone form.