Healing magic replenishes HP by using mana which is the fundamental force that makes up this world. Healing magic recovers a certain amount of HP in the target, and this helps recover the target's energy which was lost due to heavy adventuring. In addition, the power of this magic is strong enough to affect the target as well as the caster. The caster who has finished preparing to cast this magic is protected from external attacks by the power of magic. This effectiveness and the experience points from this magic increases when your life is low, so proper timing would be helpful. But you must remember this one thing - that the magic only recovers HP, it cannot treat wounds.
Healing ranks up to level 4 require using the healing spell many times, often thousands of times. The easiest and fastest way to fulfill this requirement is to find a Dark Knight and request that he or she sit beside a mana tunnel anywhere in Iria. During the day, the mana tunnel will keep any player's MP at maximum. While a Dark Knight's health begins to drain from extended transformation, a healer would have all the MP he would need to extensively train the healing spell on the Dark Knight. In early ranks, prior to Rank A, it may be difficult for a healer to keep up with a Dark Knight's rapidly draining health, but Rank A and higher healers can easily heal faster than a Dark Knight can drain HP, making this an excellent method of training for your master healers in training. Note, it is advisable to disable your computer's Nagle algorithm to maximize your system performance on Mabinogi. This will allow execution of skills faster, making this sort of training more effective.
For skill ranks beyond rank 4, healing requires the use of the skill on players in deadly status. The easiest place to find such players is in Tir Na Nog where players often training Windmill on zombies fall deep into Deadly status.
You can also heal a Dark Knight who's in their Dark Knight Disarm state.