For better or worse, this is an area I excel in. I say for worse, because there are times when I have the conversation with my student and the words come out of my mouth, "You wont need this much in real life, but we are going to learn this so you have a full understanding of it."

An example would be in graphing. The most useful way in many situations to graph something, is to use the format slope intercept y=mx+b. Now you could also graph in a point slope form, but it is MOST of the time not as useful, and does not have nearly as many real world applications for the common person.

I think you are right that it takes a lot of skill to break something down and explain it to another person. I do not think however, a person needs to be an expert on every aspect of the subject, to begin teaching it.

When talking about variables in algebra, I explain how variables could be anything and use food and party guests as an example often. "So if we have 16 guests coming, and each guest is going to eat 2 slices of pizza, and each pizza has 8 slices, how many will we need?" I will create a problem, like above, where the answers will be small and with whole numbers to learn. Talk it through logically, then, create the problem algebraically. 16 x 2 = 8x. Then once again, solve the problem step by step.