The most difficult way of producing a translation is doing a tribal translation. This method goes all the way back to Ulfilas, a missionary and Bible translator from the 4th century who translated from Greek into Gothic. Ulfilas probably learned the language growing up, but since the Goth language had no written form he had to invent an alphabet for it.
In the 19th century, Adoniram Judson translated the Bible into Burmese. The language already had an alphabet and literature, but Judson had to write his own grammar and dictionary! He then used national informants to produce his translation.
The process that must take place when a missionary linguist translates the Bible into an unwritten tribal language is more complicated than either of these examples. Before producing an alphabet, the missionary must first determine the spoken structure of the language. He (or she) does this by mapping the phonemes. A phoneme is a basic unit of sound. Each language has its own set of phonemes, which then make morphemes.
A morpheme is a basic unit of meaning. It may or may not be a word. For example, "change" is a word that is also a morpheme, but "-ed" is a morpheme that is not a word, yet has meaning. Therefore, "changed" consists of two morphemes. The missionary linguist must map the morphemes of the language, after which he may finally work on his dictionary.
The missionary linguist must then map the grammar of the language. At this point, realize that there is no such thing as a "primitive language." A tribal language can have a very complicated grammar. The missionary must listen carefully to the tribal language, learning it from scratch. At the same time he must develop an alphabet and promote literacy so that the people can learn to read the Bible.
As the missionary learns the language he can record various traditions, legends and stories, which help with the dictionary and grammar. He can also begin telling Bible stories, which the tribal people will respond to, thus helping the missionary to build a vocabulary for the eventual Bible translation. (To this point I'm simplifying quite a bit.) These stories can then be written down to teach the people to read and write.
Let's say that after a few years the missionary linguist has been able to translate the book of Mark. He then has to check the translation, first of all with the tribal members to make sure they understand it. Another way to check the translation is by doing what is called a back translation. This is when the missionary retranslates his book of Mark back into English so it can be checked by translation consultants.
The process is long and complicated, as you can see. I greatly admire the missionary linguist, these heroes of the faith who live in primitive conditions while reducing a language to writing for the first time in history, and providing a tribal people with the Word of God so that they can receive God's precious Son and eternal salvation.