Took me some time to finally be able to do it. Lots of practice.
just concentrate. it's the same as trying to move your ears or pectoral muscles, all you need to do is try to concentrate on moving that specific muscle.
You have to bring both of the eyebrows inward first, like you're concentrating really hard on something. Then try and move one of them upward. I can only do my left one, for some reason.
I can do both eyebrows. Proof inbound.
There we go: