The CDC has specific guidelines on how to properly clean most cloth and fabric masks.

If using a washing machine:

Include your mask in the regular laundry
Use regular laundry detergent and the warmest appropriate water setting for the cloth
Lay flat and let dry completely, or dry on the highest heat setting

If washing by hand:

Check the mask's label to see if bleach is recommended for disinfection. If bleach is suggested, go for bleach containing 5.25% – 8.25% sodium hypochlorite.
Mix a bleach solution with 5 tablespoons (1/3 cup) of 5.25% – 8.25% bleach per gallon of room temperature water or 4 teaspoons of 5.25% – 8.25% bleach per quart of room temperature water
Soak the mask in the bleach solution for five minutes
Discard the bleach solution down the drain and rinse the mask thoroughly with cold water
Lay flat and let dry completely, or dry on the highest heat setting

While there are some other cleaning methods floating around the Internet that suggest sanitizing face masks by sticking them in the microwave, oven, or a pot of boiling water, Carolyn Forte, Director of the Good Housekeeping Institute Cleaning Lab, doesn't recommended them since they are nowhere near as effective as standard washing and drying.

How Often You Should Clean Face Masks

There aren't any hard and fast rules regarding how often you should wash face masks because it's really depending on the frequency of use, according to the CDC. If you want to be extra cautious or think there may be a possibility that someone is symptomatic, or has sneezed or coughed within close proximity, clean your face mask after each use.

source: https://www.goodhousekeeping.com/health/a32173021/how-to-clean-cloth-face-masks/