I have been a Sunday School teacher for about five years now. I am 27 and am married with three kids. I absolutely love to teach these sort of classes and do have a lot of advice because it's definitely not just something you just walk into. It is possible to have it go badly, believe me. And it is possible to have it go wonderfully. I taught Gospel Doctrine for 3 years then Sunday School Class for teenagers for the last 3, btwn ages 12-17.
For my Gospel Doctrine class:
One big thing is to definitely go by the manual. You do need to know what the lesson consists of a lot. There shouldn't be a lot of winging it. It should be an hour of material covering. With a properly prepared lesson, the spirit only then can help for it is there only after all you can do, not there when you decided to not fully prepare a lesson because you wanted to let the spirit guide you.
So, with a prepared lesson, make sure that the lesson has preplanned questions. Your class make-up should be considered a great deal. If it's a lot of talkers, then you're GOLDEN. Just pray they aren't the kind of talkers that really go off into tangents because this does or can drive the spirit away. But if you feel the tangent is something worth exploring (feel=spirit), then by all means explore it, then return. If no one raises their hand, then call on someone. Class involvement is very important. PLEASE- Do not be a teacher that feels like they need to talk the whole time. And if the class answers a question, try not to answer it yourself, just use it as another opportunity to involve the class, simply say "well, who here can answer that for them." You'd be surprised how well class will go when this happens. Then simply get back on track.
If you're going by the manual, and you see things that don't suit your personality, don't use them. For example, I don't exactly like to read every scripture reference in the manual, so I may use just one or just paraphrase it and get to the point. Many times, these scriptures are just to get to a point, and reading it may be lengthy and you may lose class member's attention if you read it. So, just summarizing and moving to the point is the best thing.
Other parts of the lesson, I will not even do cause I just think other things are better for the class. Just know that at the beginning of the lesson, it even states that you should use your best judgement on how much of the lesson you should use.
QUESTIONS are the key though. But make sure, you have an answer, and if they can't get it, then give hints if you have to and keep asking for hands. In order to fill a person's brain, you must first squeeze it first. When everyone's brain is squeezed, then you'll be amazed how accepting they are of the information you filled it with.
Nobody wants a teacher that just teaches and teaches. You ask, then you fill. For example, if I told my kids all the time, Don't cross the road, this may not be as effective as if I said, Why do you think we shouldn't cross the road. Then they say, well, hmmmmmmm. OHH, cause the cars will hit us. Then you say, that's right, so please don't cross it, OK. Again, squeeze, then fill.
Lastly, stories are good too. You may find a question in the manual that is kind of vague (I promise, they're there). In these cases, sharing a personal story to help clarify is always nice. If they're able to understand the question just fine and participate immediately, then share the story in the end after the class in finished. Try to add some funny stories as well. Keeping the class' attention is priority one in order for the spirit to stay. Funny stories usually do wonders for this. Good luck and I hope this helped, Kenny