You may be tempted to throw our English alphabet at them, but don't be too hasty. First of all, English spelling makes no sense. Secondly, we don't use all the same sounds in our language as they do. So what can be done? This is one of the things we are studying right now during our linguistic training. The technical name for what we are currently involved in is "phonemics", and it is something that they introduced all of us third semester students to last semester. Now we are studying it more in depth, with the final goal of church planting still in mind.
But how can there be a mature church without believers studying God's Word? And how can they study without the translated Word of God? And how can the Word be translated without a written language? So then translation comes by writing and the written form comes through phonemics. I will spare you the technical details of exactly how a missionary takes the phonetic language data that he has collected and forms an alphabet that is simple to read and write. Without a well thought through alphabet, you would end up with a spelling like English, which is unpredictable. If the missionary has done a good job in his phonemics, any word should have a single, obvious spelling.
Here is a random thought about how crazy our spelling is (no wonder it takes kids so long to learn to read):
Did you know you can spell the word fish like this --> ghox? Look where the sounds come from. We get the "f" sound from 'enough'. The "short i" sound comes from 'women'. And finally, the "sh" sound is extracted from 'anxious'.