Foundation first
Foundation is designed to even out the whole complexion with a relatively thin layer over the face. Its coverage is usually lighter than concealer, and its goal is to even out texture and tone, not to fully cover specific concerns.
Concealer for specific areas
Concealer has heavier, more concentrated coverage and is used in small amounts on specific areas: under-eye circles, individual blemishes, or any spot that needs extra coverage after foundation.
The right order
Apply foundation first over the whole face, then check where you still need extra coverage and add concealer on top only there. Applying concealer all over before foundation usually wastes product and makes makeup heavier.
Choose a concealer shade for the job
To cover dark circles, a shade slightly lighter than your skin brightens the area. To cover a spot, a shade that matches your skin exactly is less noticeable than a lighter one.
Do not forget to set it
Concealer without a light setting powder on top can move or settle into fine lines over time, especially under the eyes.