What preparations are you taking about? Healing physically? Emotionally?
I view and use reiki from spiritual point of view. Meaning whatever happens in the body is the result of the mind/spirit. Reiki will be able to help you heal emotionally, ease suffering. You can send reiki even to your past events, to heal your past traumas. Yes, reiki will help to restore balance in your body. But as any alternative medicine methods, healing will be very subtle and it might require some time. Don't expect overnight healing.
The beauty of Reiki is that it is always beneficial and cannot do any harm. Try it out.