Here's the official Apple Information on how to do this:

In case of emergency, use your iPhone to quickly and easily call for help and alert your emergency contacts (provided that cellular service is available). After an emergency call ends, your iPhone alerts your emergency contacts with a text message, unless you choose to cancel. Your iPhone sends your current location (if available) and—for a period of time after you enter SOS mode—your emergency contacts receive updates when your location changes.

Note: If you have iPhone 14 or later (any model), you may be able to contact emergency services through satellite if cell service isn’t available. See Use Emergency SOS via satellite on your iPhone.

Simultaneously press and hold the side button and either volume button until the sliders appear and the countdown on Emergency SOS ends, then release the buttons.

Or you can enable iPhone to start Emergency SOS when you quickly press the side button five times. Go to Settings > Emergency SOS, then turn on Call with 5 Presses.