From my experience concealing under eye is the most critical part of any makeup. The eye issues show up more prominently than anything else on the skin. In case of discoloration or darkness it becomes even more difficult to cover it all up without looking cakey. And the trick here is to, not overdo it. Even if you are applying several layers, each of them should have merged seamlessly. Its also important to choose the right product for you.
First I apply few dots straight from my Tarte 24hr concealer, under my eyes in a circular form. Then with a sponge I blend it seamlessly. If you need more then apply and repeat. However, I wouldn't recommend too much product, if you want to avoid creasing.
Once everything has blended, use a highlighting pen like either Clinique, L'oreal Age Renewal or YSL Touche Eclat and apply a few stroke