how do you double tap

Check that you have the latest version of iOS on your iPhone 8 or later. · Go to Settings > Accessibility > Touch, and tap Back Tap. · Tap Double ...Use Back Tap on your iPhone - Apple Support

How to Turn on Double Tap on your iPhone

1. Check that you have the latest version of iOS on your iPhone 8 or later.
2. Go to Settings > Accessibility > Touch, and tap Back Tap.
3. Tap Double Tap or Triple Tap and choose an action.
4. Double or triple tap on the back of your iPhone to trigger the action you set.

You can even set a double or triple tap to trigger an Accessibility Shortcut to quickly access features like AssistiveTouch, Siri Shortcuts, Magnifier, Reachability, and VoiceOver.

https://support.apple.com/en-us/HT211781