On high heat stir the condensed milk, butter and sugar continually. (Add whatever flavors/colors you desire.)It should be hot enough that it is bubbling without boiling. I use a wooden spatula for stirring. As time passes the mixture thickens, and you start to see the pan bottom as you stir. When the mixture slowly spreads out after being pushed to one side it is ready for spreading. Periodicaly check for how chewy it is by putting a little on a teaspoon and cooling in the freezer.
Spread out mixture on buttered surface, spread with buttered rolling pin and then score and cut into squares while still warm.