Quick and easy method:  

Heat 1 C of your favorite milk in a sauce pot to a simmer (or heat in the microwave for about 30 seconds). Whisk in 2 tablespoons of the hot chocolate mix and continue to gently simmer for 30 seconds. Pour into your favorite mug and top with a marshmallow. Enjoy!

For a rich and delicious treat:

Heat a ¼ C of water in a saucepan to a simmer.  Stir in ¼ C hot chocolate mix.  Whisk smooth.  Pour in ¾ C of your favorite milk, whisk and return to a simmer.  Allow to simmer for 30 seconds, whisking constantly.  Pour into your favorite mug and top with a marshmallow.  Enjoy!