From the United States Occupational Safety and Health Administration's respiratory protection training videos.
When you must wear a respirator to protect yourself against airborne contaminants in your workplace, it is very important to follow proper procedures for putting it on and taking it off.
You have a right to a safe and healthful workplace. Whenever respiratory protection is required to be worn to protect you from hazardous airborne contaminants, your employer is responsible for implementing a comprehensive respiratory protection program and providing you with an appropriate respirator. This program must include training, medical evaluation, and fit testing.
Note: The first 4:40 minutes of the video cover the caveats, donning and doffing, and how to do a seal check with an N95 filtering face piece.