How exactly should you care for a person with possible head neck or back injury?

Get help. The main problem with bystanders and neck injuries is a desire to help them yourself. First - call for qualified help (ems, ambulance, 911). Sometimes patients have an unstable neck injury that can be made worse by moving them. Let the appropriately trained people put the person in a neck brace and then take them for x rays.
Head or spinal injur. Need to get mris to visualize exact problems, so that advice can be given.