Put on your foundation, and whatever it doesn't cover can get a layer of concealer. If it still doesn't cover, try a little more foundation on top of the concealer. Let's not be mysterious here: concealer is just a thicker form of foundation. The tricky part is getting coverage without winding up with a big lump of makeup stuck to your face. Powder can be helpful: sometimes, it's easier to start with a layer of powder over the offending spot, then add foundation on top of that. The powder thickens the foundation a bit, giving you a little more coverage.
Concealer can be used differently depending on what you want to conceal. If you're trying to tone down the dark places under your eyes, start with foundation and see how that does. Sometimes, concealer used under the eyes turns you into a raccoon face because it's just too light. Because of the way our faces are built, everyone should have a little shadow under the eyes. So go easy on the makeup, even if you had a hard night. Start instead by trying to reduce the puffiness. Apply a tightening eye cream, or lie down with wet teabags on each eye: the tannins will help reduce the swelling. Then, if you still look like something the cat dragged in, put on a thin layer of foundation.
