It's much more difficult than it sounds. It means much more than being "nice." It means love, yes, but mercy is a special modality of love. To be merciful, one must shape one's actions according to the needs of others. Mercy seeks to serve the needs of others. It means to approach the other person with an awareness of his or her weaknesses and frailty, and be an instrument of help. Sometimes it means building up the person. Sometimes it means trying to help a person correct a problem. Sometimes it means comforting the person. Sometimes it means suffering with the person.
But mercy never dismisses the needs of the other. Mercy does not dismiss the sins of the other. A doctor would hardly be considered merciful if, finding a treatable tumor inside his patient, he were to ignore it out of fear of upsetting the patient.
At the same time, the doctor usually doesn't pull out a scalpel and immediately begin cutting the tumor out. He considers the best method of healing the patient, taking into account all of the patient's needs. He also does well to be sure that his diagnosis is correct. So it is with mercy. Mercy does not always make the other person feel good. But the focus and shape of a work of mercy is the other person's good, not my desire to feel proficient in wielding a moral scalpel. That applies to the whole realm of the works of mercy. Mercy does not consist in the pleasure I derive from building social or spiritual systems of providing for the needs of others.
Mercy means that I subordinate my actions, my talents, my mind, and my heart to real needs of the person in front of me. It is a kind of self-emptying, in imitation of God who "emptied Himself" and took the form that would touch our needs from within our history. It is an abandonment of self to the person in his or her need.
This is not easy. But it is beautiful. Because the person in front of me--my child, my wife, my friend, the person in the store, the person who just posted on Facebook--that person, in his or her concrete need, is Jesus.
