Whether you have blemishes, scars or dark circles under the eyes, you need to know how to apply concealer makeup properly. Choosing the right concealer for your skin can be a very tricky process, but if you don't know how to put it on so that it looks natural, you will have wasted your effort. Fortunately, it is easy to learn and there are plenty of available resources to help you.
The first rule of good concealer is to use it sparingly and gently. As you get more comfortable and experienced with it, you will get to a point where you know exactly how much to put on and will do so in seconds.

But you'll always have to be gentle, especially when using concealer under the eyes. This is the most delicate skin on your body and it's where age will show first, so you want to protect it.

Many makeup artists advise using a makeup brush to apply concealer under the eye, as well as on pimples. If you go this route, you should take care in choosing a brush that works best for you. Sephora has a good selection and the sales assistants can help you select a good brush.

However, most women find that the easiest, most comfortable and most accurate method of applying concealer is to use the fourth finger, which applies the least pressure, and pat until the makeup is blended.

Always start with less than you think you need. You can add more if you think it's necessary, but definitely start light. Otherwise, you might end up looking cakey or crepey.
Mastering the Technique

Once you begin to feel comfortable with the basics of how to apply concealer makeup, there are some other things you should know. For pimples, you want to apply the concealer over the foundation. Otherwise, you run the risk of wiping the concealer off. You can also achieve a more natural look this way.

If you're trying to cover up circles and having a hard time making the skin look smooth, apply a light moisturizing cream before you apply the makeup. As with the makeup itself, you want to gently pat on the cream. What a lot of women don't know is that you should apply the cream about an inch below the eye, rather than right on the area to be concealed itself.

The reason for this is that the lower lid is very absorbent, so that cream will effectively "travel up" to where it's needed. When you put it directly on the lower lid, you run the risk of creating puffiness. Allow the cream to absorb, then gently blend in the concealer. If you find the makeup is flaking through the day, you can try adding a bit more moisturizer on top. This will create a more translucent look. Try to resist adding powder. Yes, it does set makeup, but it may make any flakiness look worse.

For pimples or other facial blemishes, however, a powder finish will help tremendously. It will keep the makeup in place and maintain a more complete blend. Be sure to dab it on lightly, or else you may end up with a distinct white spot. That is better than a red pimple, but defeats the purpose of masking the blemish.

As to how to apply concealer makeup to mask eye bags, you want to go against instinct and pat the makeup into the shadow underneath the bag. If you apply makeup on the bag itself, you will end up drawing attention to it.
A Final Note

Whatever you are trying to conceal, begin the process by working from the center of the blemish and then carefully feathering outwards. Never rub! If you aren't pleased with the look, ask for help from a makeup expert. You may be surprised at the extra tricks you can learn.