The process to almost 9 years and much hardship. The monks that tried this were Shingon Buddhists, a major Japanese sect. It is believed that many hundred of monks tried this but only a few succeeded.
First let's discuss the why. The Sokushinbutsu and other Shingon Buddhists believed that life is an illusion and if you do not break from the illusion you will continue to be reincarnated in it. If you do break from the illusion you will become one with the enlightened being or Buddha. I have been taught that only death is real, different strokes I guess. An important aspect of the Shingon sect is the denial of the importance of their physical selves, often by physical degradation. It is common for Shingon monks to meditate under freezing waterfalls. The Sokushinbutsu achieved this by undergoing the long an difficult suicide process.I'd just shoot myself in the head and have a friend take pictures.
Or at least that's what I can get from reading a couple of websites. There's a lot I don't know about Buddhism, like how can a peaceful religion justify killing the shit out of people with their bare hands, death point strikes and whatnot? I really don't know or care much about any religions, but I hope I didn't offend any Buddhists reading this.
Now for the how. From start to end the entire process of mummification took 3000 days or almost 9 years. It was undertaken by elderly monks near the end of their lives anyways.
For the first 1000 days the monk would only eat nuts and seeds and subject himself to a very strenuous exercise regime. This reduced the monks fat deposits, and fat is easily decomposed.
The next 1000 the monk only ate bark and roots of pine trees. Near the end of this stage he began drinking tea made from the sap of the usrushi tree (Toxicodendron verniciflum), a poisonous sap usually used to lacquer bowls and furniture. The tea would induce vomiting, sweating and urination reducing his body moisture. It also made his body so poisonous that maggots couldn't eat it. By the end of the first 2000 days he had almost no body fat from the exercise and diet, was chronically dehydrated and poisonous from the urushi.
He would then be entombed in an airtight chamber with just enough room for him to sit in the lotus position. There was an airhole and a string tied to a bell outside. Everyday he would ring the bell to tell his crew that he was still alive. When the bell stopped ringing they would seal the airhole, leaving the monk to desiccate for another 1000 days. At the end they would unseal the tomb. Most of the they found a rotted body because of the bacteria that lives inside of humans. But every once in a while the monk would be successful in his mummification. The mummy would then be removed from the tomb and placed on display in the temple and viewed as a Buddha. Those that failed would be admired for their determination and endurance and retombed (dibs on band name).
Unfortunately the Japanese government pussied out and outlawed the practice in the 19th century. While suicide remains legal to this day there, assisted suicide and exhumation are not. There are about 16-24 known Sokushinbutsu in Japan and several can be seen to this day at various temples.
Here is a site with more information about the subject: http://www.sonic.net/~anomaly/japan/dbuddha.htm
All this talk about the far east is getting me even more pumped up for the new DIM MAK