This isn't a step-by-step walkthrough. In a real emergency, call 999 and the call handler will guide you.
If they've been injured (a fall, a crash, a sports injury) and are breathing normally, leave them as you found them and keep their head still. Only roll them onto their side if they vomit, their breathing becomes noisy or blocked, or you have to leave them alone. If you do roll them, keep their head, neck and back in a straight line — get someone to hold the head if you can. If they're not breathing normally, or only gasping, don't use the recovery position: start CPR.
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Babies under 1: cradle them in your arms with their head tilted downwards. Call 999 and keep checking their breathing and how responsive they are until help arrives.