Pour milk to a thick bottomed sauce pan over medium heat. Add sugar to it.
Whisk or keep stirring with a spatula until it thickens. Take care to stir it continuously, so it doesn't stick to the bottom of your pan. This will take about 15-25 minutes. Milk will be fairly think at this time.
Switch flame off and add a pinch of baking soda to this mixture.
Keep stirring in intervals as the milk is cooling. This will prevent the condensed milk from forming a layer on top.
Allow to cool down to room temperature. Store in a clean jar!