You may be thinking to yourself that this doesn't make any sense. How can you teach something to someone that you don't already know? The answer is simple, you start by learning yourself.
Teaching someone else something new is not about being exhaustive and not always about being comprehensive. It's really about setting them up to understand the problem domain and to be able to intuitively answer, or find the answers themselves, to questions about that domain.
Here are some questions you can ask yourself in order to prepare for teaching something you don't already know about.
What is the context?
When do people use this approach/tool/etc? What's different about it than it's alternatives? Does it require a certain platform, design, or methodology? Who is it's target audience? How is it consumed? How is it distributed? What's it's history? What problem is it trying to solve?
These are all basic questions that help you understand why this thing exists. Understanding it's purpose is one of the first steps in understanding how to use it.
Almost everything you interact with makes some type of assumption. A t.v. assumes you can see or hear. A microwave assumes you have electricity. A car assumes you have have a license.
What assumptions exist in your problem domain? How have they influenced the decision making in that domain?
What are the tools?
Are there particular tools that a "standard ..." would use? For example a plumber uses a monkey wrench. A Java developer uses Eclipse. A .Net developer uses Visual Studio.
What are the tools that are used to aid in solving problems in that domain? How are they used and in what context? Understanding what tools to use will save you time that you would otherwise be spending re-inventing the tool and it's use-case.
What are the gotchas?
Every problem domain has some area or areas that people get hung up on. C++ has pointers. Object-oriented has polymorphism and inheritance. Java has the final keyword and it's many different meanings.
These are good places to start. Understanding where people get hung up is usually a good place to learn what people are missing or misunderstanding that has caused them to get hung up.