Contouring how to?
Well, prettyinpa pretty much sums it up. Where and how you apply then depending on what do you want to do with your face (narrower in general? wider on top narrower on bottom? higher cheekbone? sharper face?) and you can use anywhere from one to six or seven extra product to achieve that (foundation 1 shade lighter, 1 shade darker, luminizer, blush, bronzer etc). Since we are not suppose to post outside links, here are images of before/after contouring I found online. You can see the face shape is slightly different.
For me, I have defined cheekbone but want my face to look narrower, so I just sweep bronzer or slightly darker powder from above arch of eyebrows down to the chin. Some people want more defined bone structure, so put liminizer from eyebrow to cheekbone in a C shape where the temple is the middle of the C, then blush on the cheekbone and bronzer under it (the hollow when you suck in your cheek), then blend. Those are the common way, you should experiment to see what you want, and you can totally get fancier and go into costume make up territory where you look always like a completely different person (there are some pretty cool tutorials of those online, looking like Lady Gaga, Marilyn Monroe, Michael Jackson etc).