
This is one of the most important and fundamental parts of doing your makeup.... and can also be one of the hardest.
Contouring is the application of makeup to highlight, define and/or mask the natural shape of your face. Contouring helps to define your cheeks, slims your nose and even shorten your chin, if need be.
The first step is to decide how your face is shaped and if it is proportionate. Do you have a long face? A large forehead? Round cheeks? Wide nose? All of these can be masked with the proper application of a bronzing or highlighting powder. A product I love that I have found works on most skin types is MAC's Sculpt and Shape Compact.

After you have applied your base and powder take your brush, this can be either your blush brush that has been properly cleaned off, or a highlighting brush such as MAC's 168 Brush ($32)- I really like to use the brush that came in my Pixi blush, flat and wide, perfect for highlighting!
Using a pale powder has always been my first choice for highlighting but liquid highlighting makeup also works well, it gives a great finish and will really catch the light because of the light reflecting particles in it, giving the softest look. Highlighting brings the area forward.Softly apply the powder to the bridge of your nose, just above your cheek bones and eyebrows. Here is a great chart showing the areas you want to highlight in blue and contour in brown.
When contouring you want to focus on the areas that you want to "send back" or define such as the sides of your nose to slim, the hollows of your cheek to create a chiseled facial structure and if you have an extra long face and want to minimize your chin just apply a small amount just underneath your chin slowly and evenly blend upward.
To contour your forehead apply your contouring makeup to both of your temples and blend well. This will give you forehead a slimmer look.
The goal is to create "shadows" that give the illusion of a smaller, more defined feature.
picture courtesy of makeuptalk.com