That's a great question TMK.
So the Maasai are already monotheistic. Their word for God is Nkai or Ngai. So that is a plus. When you ask a Maasai about the character of God or how to reach him, they don't know. They don't have a working theological structure for those kinds of things. That being said we literally start from "In the beginning."
Oral tradition is how things are passed in the Maasai culture. So we tell them a lengthy story from Genesis to Revelation, focusing on Salvation through Jesus. It goes like this:
We lay a framework for creation, the character of God, and his expectations of mankind. We talk about the enemy of God, Satan, the fall of man and the consequences of it.
We then go into the concept of atonement (although we don't use that word) through sacrifice. We talk about various characters of the Bible as examples. Once that concept is grasped, we go into the depravity of man (of course we don't use that word) by showing that even though a man can sacrifice many, many animals on his behalf, his heart is still unchanged.
So by that point we established a clear contrast between God and men, established the fact that men need to reach God and that God wants to reach men, and finally established the fact that men simply cannot make it happen.
Then we lay groundwork for Jesus. We talk about who He is, what He did, and the power that He has to not only heal and save but to forgive people of their wrong-doings. We talk about the crucifixion in the context of the continued plan of God for the salvation of everyone. We establish the resurrection as a fact that proves that Jesus is God, that He can save, and that He can be trusted. We lay the groundwork for eternal life, for eternal damnation, and center everything that is discussed from that point around Jesus. It takes a good while to go through that, but they are accustomed to long stories and they enjoy it. Also, when we extend the invitation to them, many of them respond.
At that point, it becomes a game of discipleship. Our translators are pastors and generally, they return to plant a church.