Before you ever need to know, learn how to Use a Stun Gun.
Practice with and learn about this weapon so that you know what to do in an emergency.
Be prepared. Have it in your hand if you think you might need it. It's no good to you in the bottom of your purse.
Read the instructions thoroughly and completely understand how your weapon works.
Choose a stun gun that suits your size and where you'll likely use it at home, at work, outside?
Aim for contact points of maximum effectiveness - upper hip, below the rib cage and upper shoulders. Not only are these body parts nerve centers, they are also not as fast-moving as - for example- an arm or leg.
Understand and respect the safety features and rules.
Keep your stun gun in good working order.
If 'surprise' is an important part of your defense strategy, practice preparing to use your weapon without telegraphing your intent.
Don't tell a potential assailant that you're carrying a stun gun. It cancels the stealth advantage- particularly if you have a cell phone stun gun or a flashlight stun gun
Follow common sense safety behaviors and practice attack avoidance techniques.