"There is this specific statement in the Padma Purana: ďA person who honors the prasada and regularly eats it, not exactly in front of the Deity, along with caranamrta [the water offered to the lotus feet of the Lord, which is mixed with seeds of the tulasi tree], immediately can achieve the results of pious activities which are obtained through ten thousand performances of sacrificial rites."
Nectar of Devotion, Chapter 9
There are many Vedic injunctions for the process of taking prasada:
One should take prasadam in a peaceful state of mind. The body and dress should be clean and fresh. Five parts of the body should be damp (from washing): two hands, two feet, and the mouth.
The householder may wear rings on the fingers, and should cover one's head. One's clothing should not be damp, and shoes should not be worn. The man's sikha should be properly tied up.
It is best to sit in a clean and spacious place, preferably on an asana, with the legs crossed rather than spread out. One should be facing east or north, and should avoid eating while sitting in a vehicle.
Meals should not be taken at the sandhyas (sunrise, noon and sunset), and one should not eat before performing sandhya vandana (chanting Gayatri) or morning puja. Don't take food until the previous meal has fully digested.
Tamala Krsna: "It's a great offense if you offer someone prasadam and they refuse.
Prabhupada: Yes. That means I am not accepting you as intimate. And if he accepts, then you cannot deny his friendship."
Srila Prabhupada Room Conversation, 07-10-76, New York
According to formal protocol, those of similar rank should sit together in the same row. The senior-most person should be given all respects, and should be permitted to finish first before others finish and rise to go. As soon as one person rises, the meal has ended and others in the row should not continue eating. The senior person may also choose to sit separately, so as not to disturb or be disturbed.
Mahaprasad is non-different from Krsna, therefore prasada should be honored, not eaten. One should never consider mahaprasada to be ordinary food preparations. Before taking, one should first glance over the prasada and offer respects, remembering that prasadam is the mercy of the Lord. One should ask Krsna's permission to eat, reciting the Prasada Sevaya prayer (sarira abidya-jal). One may also recite the Jayadhvani prayers and according to Hari Bhakti Vilasa, one should chant the Gayatra mantra then chant one's mula mantra seven times over the prasadam. (See link below for prasada prayers.)
The householder should serve prasadam as follows: First, guests should be served, particularly those who are spiritually qualified persons, such as sannyasis. Next, one should serve the elders, married daughters, pregnant women, sick persons and children. The grhasta should also feed tied-up cows and horses before taking his own meal.
An initiated Vaisnava should serve another initiated Vaisnava. The server should be clean and peaceful, performing his duties quietly and efficiently. Loud noise and frenetic activity should be avoided.
One should serve prasada with the right hand. Neither the server's hands or the utensils should ever touch the plates or hands of those who are eating, as this will contaminate the server and utensils. If this happens, hands and/or utensils should be washed before serving resumes. Serving vessels should not touch the feet. The server should in no way contaminate the prasada or utensils by touching his mouth, feet, hair, lower body, nor should he yawn, sneeze, etc.
Prasadam should be served onto a plate, and the plate should not be kept on one's lap. Preparations should be carefully dropped onto the plate in a free area, and spoons should be used to serve most items, except sweets, water or dry items. Foods should be taken from the plate, not received directly into the hands and placed in the mouth.
One should sit quietly while taking prasada, not discussing material affairs. Those seated together should not touch one another while eating. One should only use the right hand to place foodstuffs in the mouth. Avoid making unnecessary noises while eating. One should tear large items with the fingers of the right hand, not with the teeth, then small pieces should be placed in the mouth. One should avoid eating with the left hand, which is contaminated due to other uses.
The householder and his wife (or others who are serving) should be last to eat, and should stop taking prasada themselves whenever necessary in order to attend to guests (or wait until others are finished to begin taking themselves). When guests finish a portion, they should quickly be offered more of it, and should be permitted to have all they wish. All persons in a group should be offered the same items. The food should be served while still hot, if possible. Guests should never be left with an empty plate except at the end of the meal, or when they have exclaimed they can take no more. The server may describe the wonderful qualities of a preparation as he offers it, to encourage the guest to eat more.
While the server encourages guests to continue eating sumptuously, one should not overeat. Excessive eating causes discomfort, sickness, and decreases the lifespan. It interferes with one's devotional activities, causes one to be unregulated, and generally encourages sinful behaviors. Too much sense gratification, over-dependence on another's mercy, and artificial standards of living are detrimental to spiritual advancement.
It is said that one should fill half the stomach with food, one quarter with water, and one quarter with air. The order of foodstuffs eaten is also important. The Hari Bhakti Vilasa states that one should begin by eating a little sweet food, then take salty and sour items, and then pungent and bitter items. Different customs are followed in different regions of India. In Bengal, for example, it is the custom to take bitters first, and end the meal with sweets. Generally, liquid foodstuffs should be taken at the beginning and end of the meal, and solid foods should be taken in the middle.
"Caritamrta is obtained in the morning while the Lord is being washed before dressing. Scented with perfumes and flowers, the water comes gliding down through His lotus feet and is collected and mixed with yogurt. In this way this caritamrta not only becomes very tastefully flavored, but also has tremendous spiritual value. As described in the Padma Purana, even a person who has never been able to give in charity, who has never been able to perform a great sacrifice, who has never been able to study the Vedas, who has never been able to worship the Lord-or, in other words, even one who has never done any pious activities-will become eligible to enter into the kingdom of God if he simply drinks the caritamrta which is kept in the temple. In the temple it is the custom that the caritamrta be kept in a big pot. The devotees who come to visit and offer respects to the Deity take three drops of caritamrta very submissively and feel themselves happy in transcendental bliss."
Nectar of Devotion, Chapter 9
One should drink water out of a cup or vessel, using the right hand and not touching the lips to the vessel, particularly if others might drink from it. Water should not be drunk directly out of the hands. While drinking, water should not be allowed to drip onto the prasadam, which then becomes ucchista (contaminated leftovers).
After finishing the meal, one should take the plate (if disposable) in the right hand and discard it. All food remnants remaining on plates and in serving dishes are ucchista, and may not be mixed in with other, un-offered foods, or returned to the kitchen, or in any way be re-offered to the Lord.
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.