Corn flour is common in the UK, but in the United States they have "cornflour" instead. Technically, corn flour is made from the whole kernel of corn while cornflour is only from the endosperm part of the kernel. Corn flour is easy to buy in supermarkets in the UK. However, you can make your own corn flour by grinding corn meal in the blender until it becomes a fine powder, or use a spice grinder. As thickening agents, cornstarch and corn flour are almost interchangeable with an adjustment in the amount.
Estimate how much soup you want to thicken by how far it fills the pot. If the liquid comes up halfway in a 4 litre (4 quart) saucepan, you have about 2 litres (2 quarts) of soup.
Place 1 tbsp of corn flour for every 237 ml (1 cup) of soup into a small cup. If you're using cornflour, use 1 tbsp for every 474 ml (2 cups) of soup. Add 2 tbsp of cold water and combine to make a paste.
Add 118 ml (1/2 cup) of hot soup to the paste in the cup. Mix well.
Pour the diluted paste into the soup, whisking as you do it. Heat the soup to boiling. Boiling the soup takes away the raw taste of the corn flour.
Repeat the process until the soup is as thick as you like.
Decide how thick you want the finished soup to be. You might even experiment a bit with a trial run of water thickened with corn flour before you make the decision. The soup will thicken even further as it cools.
Use caution when working with boiling liquids.