HOW TO APPLY KUMKUM?.

The blessings of the Almighty is basically of two types. One form is invisible to the naked eye - in the form of Punyam, Dharmam etc, which is present with us and protects us. The other is in the form of visible prasadams - which have been offered earlier to the Supreme - and presented back to us as prasadam/naivedyam - like flowers, vibhuti/kumkumam etc.
The invisible form of blessings protects us in the realm of mind and intellect - controlling our emotions, guiding our mind appropriately in sat-vishayam etc.
The visible form - the prasadams like chandanam, vibhuti, kumkumam, flowers etc - by being worn on the face or body, protects our body from any type of accident or untoward incidents. In addition, it also brings up the thoughts about God in others, whoever gets to see such marks in our body. This is the basic reason why our Saastras enforced us to wear appropriate prasadams always - according to the religion/sect - may be thiruman, vibhuti, gobi, kumkumam etc.
Further, the Sastras proclaim that the right portion of our body (like hands/legs) is much more stronger and pure as compared to the left, and hence every one (except the naturally left handed people) must use only the right side of the body to perform any type of divine or auspicious acts.
In line with the above rules, we must obtain or receive any prasadams only with our right hand, with utmost devotion, and must wear it on our body only with the right hand (except where impracticable). The Sastras also clarify that each finger in the hand has a unique character or power and must be used appropriately.
The ring finger is supposedly possessing more power than the other fingers, and hence must be used for applying chandanam on the deity vigraham or pictures etc. The same finger must also be used for applying such prasadams in our forehead/body.
It may however be noted that, applying prasadams using any other fingers (of the right hand) cannot be categorized as incorrect or a crime. What is more important is to ensure that everyone wears some such prasadam in their forehead and assist all around to constantly dwell in the thought of the Supreme.