Teaching something you only just learned
Three months into Spanish, a friend asked me to help her start. I learned more in six weeks of teaching than in three months of study.
The best way I know to learn something is to teach it to someone else, before you feel ready. I learned this by accident, three months into Spanish, when a friend asked me to help her start. I had no idea what I was doing. I learned more in those six weeks of teaching than in the three months before.
Why it works
When you teach, you cannot hide in vague understanding. You have to say the thing out loud, in order, simply, to someone who does not know it. Every gap in your knowledge shows up immediately, usually in the form of a question you cannot answer. ((My friend asked why Spanish has two verbs for "to be". I had been using both happily for weeks. I had no idea why. I went home and found out, and now I really know.))

The beginner's advantage
A beginner is often a better teacher than an expert, at least for other beginners. You still remember what was confusing. You remember which explanation finally worked for you, and which ones did not. An expert has forgotten what it was like not to know.
When I explained Spanish numbers to my friend, I did it the way that had finally made sense to me, with my own silly ways of remembering them. She learned them in two sessions. It had taken me two weeks.
How to do it
- Find one person, not a class. A friend, a partner, a child, a colleague.
- Teach the thing you learned this week, not the whole subject.
- Write down every question you cannot answer. That is your homework.
- Be honest. "I don't know, but I will find out" is the best sentence a teacher has.

To teach is to learn twice.
something my first French teacher said, which I only now understand
It works for everything
Since then I have tried it with other things. I taught Tomás's son how to sketch a bird after my third week of sketching. I taught Ruth how to darn a sock after darning exactly four. Each time, the same thing happens: I have to understand it properly, and I find all the places where I do not.
The fear
The hardest part is the fear of being found out. What if you are wrong? You will be, sometimes. Say so, correct it, and carry on. The person you are teaching will learn that mistakes are normal, which is perhaps the most useful lesson of all.