How to color under your lines

I've been learning a lot of new stuff recently, and I realize how little I really know about how to do things, but there's an important trick that was taught to me by my friend that has been super helpful, and a must know for any Photoshop artist. How to make it so you can color under your lines.

(This tutorial assumes you have a scanned picture you want to color.)

Let me know of anything isn't clear!