Rehearse. Try to explain something, as if you were explaining it to someone else. (It doesn't have to be out loud - you can do the conversation in your head.) You can kind of tell that some parts don't work. Try again. And again. After several times, you find better ways to explain it.

Now go try it on a real person. At this step, it may work out differently than it did in your own head! Don't get discouraged by that. Keep trying.

You finally explained that in a way that someone understood? That's great! Take a moment to feel good about that. Then pick something else, and start rehearsing that.