Tegan's idea is good. There is another way too, the way that I do it. There are lots of ways actually.

I use a curves adjustment layer to lighten the entire image then hit command +j (ctrl + j on PC) (been using that command so long I don't even know what it is called when it turns black )

Zoom into the eye, change the brush size to something that fits in the eye, big if you just want to do the whole eye, or small if you want to create a catch light which is possible too. Make sure your hardness is down for the whole eye and up to create a catch light.

Then flatten it.

Okay, I like to paint and draw. I love sketching, I think that is why I like this method compared to Tegan's. They both pretty much achieve the same effect and mine takes longer... but I like the way brushing feels when I have my stylus and sometimes I get the pen really small on different adjustment layers for lighter and darker parts of the eye to pop eyes.