Add all the vegetables in a deep saucepan and add 1.5-2 litres of water and bring to a boil, then turn the heat down, and simmer for 25 minutes. You will notice that the colour will change and once you taste it, it will have a pleasant vegetable flavour.
Turn off the heat, and cover the pan for another 20 minutes to infuse. After that, strain the stock and get rid of the vegetables. You can use it now, or once it cools you can store in it in the fridge or freeze it.
Notes
Once it cools you can store it in the fridge for up to 4 days or freeze it for up to 3 months.