Dumb and obvious question, but I was never taught...

[skybie]

i use it under, cover up dark circles under my eyes then put foundation on

[Traveller]

concealer first (I use a green concealer then a skin tone to lessen redness) then foundation on top to blend it all together.

[Sopdet]

I prime, conceal, foundation, conceal, powder

[Laranna]

I usually do concealer under liquid foundation and then touch up over the top. If I'm using a mineral foundation, I'll always conceal under first.