What is the best way for a soldier to learn a subject well enough to teach it?

SGT Robert K. PRACTICE,PRACTICE,PRACTICE until he/she feels comfortable with the subject and then once again PRACTICE,PRACTICE,PRACTICE teaching it(some people cannot teach regardless of how much they might know about something)

You need to have a subject matter expertise. The best way is to gain experience in something you have a passion and interest in learning and teaching. Start with teaching your young private how to be a professional soldier and request experiences to teach others. As you gain a comfort level, you can request instructor assignment. Good luck in your new found passion.

1. Learn the task to standard - and be able to meet the standard EVERY time they do it. Take some notes about what was easy for you, and what things you had the most problems getting.
2. Write up a "lesson plan" as if the soldier was in the audience - what would THEY want to be taught, and how would they want to be taught those things?
3. Set up some stuffed animals, Barbie-dolls, action figures, or anything with faces, and pretend you have an audience. Teach your class to them, then make notes and modify what you need to, to get your point across.
3.1. Use that audience and pretend every one of them is just as stupid as a bag of hammers. Teach your class, then make notes to make sure these boxes of rocks can understand what you are teaching. Remember, these fools can ONLY learn when they are hands-on, so help them out!
3.2. Use that audience AGAIN, and pretend every one of them is a top-notch troop and has a degree in engineering and 10 years of service. Teach the class and make notes to make sure these Super-Troopers can sit through it without being bored - AND can actually learn something. Remember, these aces don't need hands-on, they want pictures or visual aids.
3.3. Use that audience one last time - and imagine that they are all O-6's and crusty Sergeants Major. Teach your class - change your underwear - and then make notes again.
4. Ask at least one friend to sit through your class. Again, make notes.
5. Have your supervisor sit through the class. Get feedback and make notes.
6. Go back and re-read ALL your notes to make sure you didn't forget anything.
7. Knock this pig out of the park and TEACH IT.

Yeah, there's LOTS of steps, but it can be done in a day (except for Step 1 - which takes as long as it takes...), and this includes time for beers and meals. The hardest part is to reach all of the students - or at least as many as possible - and to keep the rest from being bored. This works for a 10-minute refresher or an hour-long lecture.