You need the following to brew a cup of Vietnamese coffee:
A Vietnamese filter
Condensed milk typically contains sugar and is heat processed with steam. Steamed milk has different physics from non-steamed milk and tastes better when used in coffee (ask any cappuccino drinker).
Pour about 1/3 of an inch of condensed milk in the bottom of a coffee cup.
Unscrew to remove the coffee filter’s top screen.
Place three rounded teaspoons of coffee into the filter. Replace the screen and turn it until it starts till its tight.
Place the filter on top of the coffee cup. Next, pour hot water (not room temperature) into the filter until it is about a quarter full. Time it for a good 20 seconds. Make sure that the water flows through all the way within 20 seconds. If it doesn't, that means more tightening of the filter needs to be done.
Unscrew the filter with two complete turns. Note that turn it twice only.
Fill the filter with hot water. Now, place the cover back on and wait for 5 minutes. In this step, the water should be dripping and not flowing.
When the dripping is completed, carefully remove the filter (the whole filter set). It is now ready to be served. You can either stir it with the condensed milk or you can drink it as it is.