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Keep control of the class. No matter what happens you must control the class. Even if you are stumbling in your presentation, do not let some mouthy students take control of the class. They may just be trying to be funny or maybe even trying to be helpful, but you have to keep them in check.
In general, the more you prepare the worse you will do. Preparation is useful but how you prepare and how much time you need to spend on preparation is a different matter. Do not over prepare. Have a general direction for conducting the class and some alternative directions that depend on the rank composition of the class. Then rely on your experience and training. You would not have been selected as an instructor if you instructor did not think you could handle it. You will not be great the first time out, but you will improve each time you teach.
The class truly wants you to succeed. Most of them know the feeling of having to speak before a group. They know you, and they are probably your friends; so they feel for you. They will admire your courage and will be on your side, no matter what happens.