NIOSH Publication No. 2010-131: “How to Properly Put on and Take off a Disposable Respirator,” includes these respiratory protection 101 basics:
Wash your hands thoroughly before putting on and taking off the respirator.
If you have used a respirator before that fit you, use the same make, model and size.
Inspect the respirator for damage. If your respirator appears damaged, DO NOT USE IT. Replace it with a new one.
Do not allow facial hair, hair, jewelry, glasses, clothing, or anything else to prevent proper placement or come between your face and the respirator.
Follow the instructions that come with your respirator.