After you add the lemon juice to curdle the milk, you must keep stirring the milk continuously until it’s time to take the pot off the heat. Add the sugar to the pot and continue stirring and scraping the bottom of the pan. The sugar will make the milk stick more to the pot and so you must keep stirring continuously.
As you continue stirring, you will also notice that the color of the mass changes as the milk gets caramelized and the mass has a thick pudding consistency. Reduce the heat to low.