For the basic information of the bandhas, read this article: bandhas.
In Sanskrit, Maha means great, superior, higher.
Bandha means to hold, tighten or lock.
Maha bandha is considered the superior of all the bandhas. This is the combination of all 3 bandhas.
According to Gheranda Samhita, verses 14 to 16:
Close the anus with the left heel, press the left heel with the right foot with effort, slowly expand and contract the perineum and retain prana vayu through jalandhara bandha. This is called maha bandha. It is superior to all other bandhas and is the destroyer of old age and death. All desires are fulfilled by the grace of this bandha.
Sit in any comfortable meditative asana, preferably siddhasana or padmasana.
Make your knees firmly grounded.
Place the palms on your knees.
Relax your whole body.
Inhale deeply through the nostrils, then exhale (quickly and forcefully) completely through the nostrils (can exhale through the mouth also).
Apply the jalandhar bandha just after the complete exhalation, then uddiyana, then moola bandha.
In the final position, keep pressing the palms on the knees, arms straights.
Maintain this position, hold the breath as long as you can without strain.
Then release moola, uddiyana and jalandhara bandhas in this order.
Then inhale slightly deeper and exhale longer.
Rest here for 3-5 natural breaths. This is one round. Then, repeat the same thing.
Start with 3 rounds and gradually increase the practice until 9 rounds.
On 3 related chakras: vishuddhi, manipura and mooladhara.
According to Hatha Yoga Pradipika 3:75
With this method, the air (prana vayu) attains absorption i.e. merges in susumna. Then (for sush a sadhaka), death, old age, disease etcetera do not occur.
And, the sutra 76: This triad of bandhas is considered to be the best of all and is practiced by great yogis. This is considered to be the means of all the hatha practices by the yogis.
We practice all the 3 bandhas together so that all their benefits are obtained.
Affects the hormonal secretions of the pineal gland.
Regulates the entire endocrine system.
Increases the level of the energy, boosts the immune system and slows down the aging process.
Each and every cell of the body is energized and rejuvenated.
Calms down the mind and increases the level of concentration.
Tranquillity of the mind will be clearly observed just after few rounds of practice.
Very good practice for the preparation to meditation.
It can fully awaken the prana in the main chakras (mooladhara, manipura, vishuddhi).
It unifies prana, apana and samana in manipura chakra, which is the culmination of all pranayamas.
People sufferinf from high or low blood pressure, heart conditions, stroke, hernia, stomach, stomach or intestinal ulcer, and those recovering from any visceral ailment should avoir this practice.
Pregnant women should also avoid this practice.
Stomach, bowels and bledder must be empty.
This is a advance practice of pranayama, so don't force yourself. Increase the time and number gradually.
Best time for this practice is the morning.
Those days you are performing these bandhas, use only sattvic food.
Practice in the guidance of a teacher.