I received an e-mail for a young man from an country in Africa who wanted to know how he should prepare to do a ministry within the mentally challenged community.  He had read this blog about The Special Gathering which is a program designed to evangelise and disciple people who are developmentally disabled.  I realized that he would be working with limited resources and that he have many limitation regarding his education.  Here is my answer to him.  

Thank you for your email.  I would begin with studying your Bible and praying for God to help you begin.  Then I would try to gather some people (one or two would be perfect) who are mentally challenged from your church, town or city. Once you have gathered them, you will need to teach them.  I post a sermon on our blog each Sunday.  You should adapt it to fit your particular needs in Ghana.  You want to sing some simple songs (about five or six).  Ask for prayer requests, then take up an offering and have prayer.  Then you will preach, teach the sermon. 
I would then have a break, maybe have something very simple to eat.  Perhaps some one from your church would be willing to provide one cracker or cookie for each person, perhaps some simple fruit (mango).  Then after that I would go over the Bible lesson that you have taught in your sermon again.  I actually tell the story two more times.  Then I ask questions, in this way, I have the students tell each other the story one more time.  Then I use a practical question that helps them to apply the lesson to their lives. 
The model is very simple to follow and I know it will work.  Thanks for asking.