I presume that they know the alphabet and can write in their native language.  In that case, start off with what they know-- request a simple paragraph about their daily life, hobbies, family, etc.  That is all you really need to do to work on writing:  give them the opportunity to write.  But make it homework. Don't waste valuable classtime with the native speaker in silently writing.  Reading also should generally be confined to outside activity-- this also gives them English practice time in addition to classroom hours.

I have a lot of my students writing me weekly emails before class now, and I can either correct it and return it with explanations in class, or send the advice by return email.