Remember that regular practice during the first 21 days is very essential, so ensure that you practice atleast once during the initial period. Your journey does not end after 21 days though, and you would need to continue daily practice even after that, but yes, its ok to miss a day or two once in a while.
Here are other links you will find useful:
Reiki Self Healing
Healing Others through Reiki
Healing for Specific Problems
Detailed Information on the Chakras
Reiki Articles
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