How should I use concealer?

I have so many blemishes on my skin that foundation or

Even when I applied a blemish-covering cushion, it didn't really cover them up.

So I bought a concealer and tried using it, but the concealer

It covers blemishes when used, but it makes the skin look too dull

The skin was expressed, so it became patchy. ...

I'm having trouble deciding how to use concealer.