Take one cup of water in a measuring cup.
Add one cup of distilled white vinegar to it.
Mix the two liquids with the help of a stirrer or spoon until thoroughly combined.
Boil water and pour it into the flask. Pour the hot water out and repeat the process one more time.
Now half fill the flask with the water and vinegar solution. Cover it and shake vigorously for about twenty seconds. Pour out the solution and rinse it with cold water.
Repeat the steps involving bowling water treatment, and then rinsing with water and vinegar solution alternatively.
After repeating these steps, fill the flask with boiling water to make sure all the residual vinegar is gone.
Turn it upside down on a paper towel to dry it.