Start by collecting your ingredients. You can keep a container in the freezer for odds and ends (like onion peels and tops of tomatoes) and just add to it whenever you cook. When I'm ready to make stock, I clean out the veggie drawer of my fridge and throw whatever looks a little wilted into the pot. Be careful NOT to put broccoli, cabbage, or cauliflower in your stock (these cruciferous vegetables are NOT pleasant additions) and if you add onion peels, potato skins, leek ends, etc., be sure to wash them well first.
Once you have a collection of vegetables and/or produce scraps, dump them into a large pot. Add just enough water to cover them, and bring it to a boil. Boil gently for about an hour, stirring occasionally. Strain the liquid into a large bowl, over a colander, to remove all the vegetable pieces and scraps. Return the liquid to the pot, and boil gently until it is reduced by half. After it has cooled, put it into freezer-safe containers and freeze until you're ready to use it.