How Do Buddhist Practitioners Dedicate Merit?

If you are able to persist in dedicating merit every day, then dedicate it in this way: With all the roots of goodness and merit I have gained from studying and practicing the Buddha's teachings today, I dedicate everything to my parents, teachers, and elders of all my past lives; to all sentient beings throughout the eons whom I have deliberately killed, accidentally killed, mistakenly killed, harmed, eaten, or with whom I have formed grudges; 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 be able to eliminate the karmic obstructions of their past lives, may their roots of goodness, merit, and wisdom all increase, may they form a connection with the Buddha early, encounter the Buddha's teachings early, and through the blessings of all the Buddhas and Bodhisattvas, soon escape the evil destinies, be reborn in good destinies, or return to the human world to study and practice the Mahāyāna teachings together with me. May they enter the Buddhist gate to practice as soon as possible, attain bodhi soon, be freed from suffering and attain happiness, and achieve great liberation. 

If you cannot persist in dedicating merit every day, then at the very least you should dedicate merit once a week; dedicating merit only once a month would be a bit long a time, and in that case you would have to dedicate all the merit accumulated from your practice over the entire month. If it is a general dedication, then dedicate all the merit and virtue accumulated from studying Buddhism and from your deeds in this present life, and also dedicate the merit accumulated from studying and practicing the Buddha's teachings in past lives. If you are dedicating merit to a specific person, you should add that person's name and dedicate the merit to that person. For someone who has already passed away, dedicate the merit so that they may soon escape the three evil destinies, be reborn in a good destiny, and have the karmic affinity to encounter the Buddha's teachings and diligently study and practice Buddhism. For someone who is still living, dedicate the merit with the wish that their roots of goodness, merit, and wisdom may increase, and the dedication should be made according to the other person's legitimate and harmless needs.