While there are a variety of tutorials online about how to edit Wikipedia—including an Orientation for students that parallels this one—it is often more effective to take students into a computer lab and do a hands-on introduction to wiki mark-up.
For the students, this:
allows them to see editing take place "live"
gives them a safe space to make mistakes
allows them to ask questions in real-time
If you are working with a Wikipedia Campus Ambassador, he or she can help you with this lab session.