If you wear your beanie frequently, chances are it's accumulated some sweat and dirt. The good news is washing our plain beanies is super easy. Just throw it in the machine with the rest of your clothes.
If you stain your beanie, you can just wash it using lukewarm water mixed with 1 tsp of laundry detergent. Once the stain is removed, let the beanie dry by hanging or by rolling in a towel. Do not iron.