Cloth face masks should be washed frequently. The California department of public health recommends after each use or daily.
Cloth masks should be laundered using detergent with hot water and then dried on a hot cycle to kill bacteria and microbes.
Basically, hot soapy water is the key. Soap is able to break down the protein coat of the virus and is very effective.
If you must re-use your mask before you are able to wash it, it is recommended to wash your hands immediately after putting it back on and to avoid touching your face.
Here are some general guidelines for cloth masks from the California Dept. of Public Health:
Face coverings can be made out of cloth, factory-made or hand-sewn, or improvised using bandannas, scarves, T-shirts, sweatshirts or towels.
The material should cover both the nose and mouth.
Ideally, face coverings should be washed after each use. Dirty masks should be placed in a dedicated laundry bag or bin.
Use detergent and hot water when washing cloth masks, and dry them on a hot cycle.
Be sure your mask is comfortable; you don't want to have to keep adjusting the mask, because that means touching your face.
Wash your hands, or use hand sanitizer, before and after touching your face or face coverings.
If you must wear your cloth face covering again before washing it, wash your hands immediately after putting it back on and avoid touching your face.
Discard or get rid of cloth face coverings that:
No longer cover the nose and mouth
Have stretched out or damaged ties or straps
Cannot stay on the face
Have holes or tears in the fabric