This method requires another Apple device besides your iPhone.
If you keep the new device in a single location, you’ll effectively freeze your location on Find My Friends.
iPhone X and later - go to the top-right corner of the screen and swipe down iPhone SE and iPhone 8 and earlier - go to the bottom edge of the screen and swipe up
Note that turning off Airplane mode or turning on Wi-Fi will update your location on Find My.
With FaceID - press and hold the side button and either volume button. With Home button - press and hold the side button.
Note that your location will be updated on Find My the next time you turn on your iPhone.