I love concealer. Sometimes, I use it instead of foundation, but it has been awhile since I've used it which means I've gotten out of the habit. Time for a refresher course for me!
Why exactly is concealer great? Well, it hides blemishes, dark circles, and capillaries, and it helps to brighten the face, giving you that alert look. Also, if your skin tone doesn't always match up naturally, the concealer helps to do that for you.
Choosing the right shade can be problematic as well as knowing where to apply it on the face. You'll want a color that is close to your skin color. Bring your foundation with you so you can compare it. Also, notice if your skin is darker in the summer than it is in the winter and adjust your color choice. A helpful tip: many places like Macy's, Norstrom, Sephora, etc. will give you a free consultation to help you choose the right color, and if you're lucky or know how to phrase it correctly, you can even get a free sample of the color. If you do, take it to a drugstore to get it cheaper! Or, go with the brand you do like!
To properly apply concealer, apply several dots of concealer under the eyes, then use the pad of your middle finger to tap it in (always tap, never rub). Apply concealer on other uneven spots on the face -- including the chin, and around the nose and mouth if need be -- and tap in. Be sure to not start with too much! It's best to layer it on as you need it. Remember to start at the center of the imperfection and feather outwards.
Concealer is last; I've been doing it the wrong way these years. The reason why it's last is because it can be rubbed off during the application of foundation. Never even thought of that! Hope this helps!