St John’s Ambulance have produced the following guide to AEDs:

The quick guide is available here: https://www.sja.org.uk/get-advice/first-aid-advice/how-to/how-to-use-a-defibrillator/

More detailed guidance on when to use an AED and on which patients is available here: https://www.sja.org.uk/get-advice/i-need-to-know/defibrillator-guide-for-rescuers/