You may be surprised to know that many people have difficulty putting their knowledge into words clearly. It is a skill that can be learned. In my teaching and public speaking experience I rely on a basic plan that has 3 steps:
1. Tell them what you are going to tell them
2. Tell them
3. Tell them what you told them
It is simple to keep this in mind for explanations or other oral expressive activities. Of course, there is more to it than that, and lots of suggestions are found on the internet. Here is a website with information about organizing a presentation. It is applicable to teaching.
http://www.presentationteam.com/News/Ti ... /index.php
You can learn and improve.