In a medium sized, heavy bottomed saucepan, combine milk and sugar. Place your saucepan over low heat, and whisk until the sugar dissolves.
Turn the heat up to medium-low, and bring the mixture to a very light simmer. Simmer your milk/sugar mix low and slow for 45 minutes to 1 hour. As it simmers, go in occasionally to skim off any bubbles or skin that forms on the top of the milk and to stir to keep the bottom from burning. *Don't mix in the impurities on the surface or it'll cause your condensed milk to be grainy!*
Cook until the condensed milk thickens enough to coat the back of a spoon. It should also be creamy and a light golden brown color. It will be much looser than canned condensed milk while it's still hot, but it will thicken as it sits and cools.
Transfer the condensed milk to a medium sized jar or bowl and cool completely before using. Seal, and keep in your fridge for up to 2 months, or in the freezer for up to 6 months.