Summarize the following:
Do not let it burn. Stir in well. Stir at least once a minute, if not more often. This will help it to thicken even more. Remove it from the heat. Then transfer to an airtight container for refrigeration or use immediately.

Summary:
Heat the milk in a heavy-based saucepan. Add sugar and baking soda to the hot milk in the pan. Keep stirring until the milk thickens. When the milk thickens, keep stirring. Once the consistency is very thick, the condensed milk is ready. Let the condensed milk stand aside for an hour to cool.