How Should Buddhist Practitioners Dedicate Merit?

If you are able to persist in the dedication of merit (pariṇāmanā) every day, then dedicate it in this way: With all the roots of goodness and merit I have gained today from studying and practicing the Buddha's teachings, I dedicate everything to my parents, teachers, and elders from all my successive lives and lifetimes; to all sentient beings with whom, throughout the kalpas, I have formed grievances by deliberately killing, accidentally killing, mistakenly killing, harming, eating, or otherwise wronging; to the successive generations of ancestors and relatives of my father's (surname) family; and to the successive generations of ancestors and relatives of my mother's (surname) family. May they eliminate the karmic obstructions accumulated over past lives, may their roots of goodness, merit, and wisdom all grow, may they soon form an affinity with the Buddha and soon encounter the Buddha's teachings, and, through the blessings of all the Buddhas and Bodhisattvas, may they soon escape the evil destinies and be reborn in good destinies, or return to the human world to study and practice the Mahāyāna teachings together with me. May they soon enter the gate of Buddhism to cultivate and practice, soon realize bodhi, leave suffering and attain bliss, and achieve great liberation. 

If you cannot persist in dedicating merit every day, you should at least do it once a week; doing it only once a month is a bit too long a span, in which case you would have to dedicate all the merit accumulated from your practice throughout the entire month. If it is a general dedication, then dedicate all the merit and roots of goodness accumulated from studying Buddhism in this present life, and also dedicate the merit accumulated from studying and practicing the Buddha's teachings in previous lives. If you are dedicating merit to a specific person, you should add that person's name and dedicate the merit to him or her. For those who have already passed away, dedicate the merit so that they may soon escape the three evil destinies, be reborn in good destinies, and, having the affinity to encounter the Buddha's teachings, diligently study and practice Buddhism. For those who are still living, dedicate the merit with the wish that their roots of goodness, merit, and wisdom may grow; the dedication should be made in accordance with the other person's legitimate and harmless needs.