Unless you're going for the cakey and crease magnifying look, follow these rules:
1. Never wear concealer alone.
2. Apply after foundation or tinted moisturizer.
Concealer are made to spot treat and applying it over foundation will not make it look blotchy.
3. Mix it up!
If you worry about fine lines, mix it up with a primer or serum first.
4. Keep it oil-free.
This is why you should apply conceal after foundation (Unless your foundation is mineral powder-based). You want to set the conceal with a translucent powder to make it last longer.
5. Play with color.
6. Correctly apply concealer.
Always conceal going down like an upside-down triangle. This prevents that raccoon eye look and it brightens up your face.
Use light concealers as a highlighter and dark concealers as a cream contour.
8. Make the most out of your concealer.
9. Blot away.
Reduce cakey texture by gently blotting the area where fine lines are most visible.