Follow the next five steps if the casualty is led on their back. If they are lying on their front or side, you may not need to go through all of these steps.
To put someone into the recovery position:
Place the arm nearest to you out at a right angle to the body, elbow bent with the palm facing upwards
Bring the far arm across the chest, and hold the back of the hand against the casualty’s cheek nearest you. Hold it there
With your other hand, grasp the far leg just above the knee and pull it up, keeping their foot on the ground
Keeping the hand pressed against the cheek, pull on the far leg to roll the casualty towards you onto their side. The top hand should now be supporting the head and the bent leg should be resting on the floor so they can't roll over further
Now you need to check the airway:
How to put someone into the recovery position