If I have a white primed face, I paint carfully around the eyeball with black, this will provide some blacklining between the eyeball and the lids. Here I try to be as careful as possible not to paint on the eyeballs, but I don't care if I paint around on the face, that will be easily corrected later. When ready I paint on the eyeballs a vertical line or a little dot, depending on the size of the eye. In your case a line would do, I think. When I paint the line, I again don't care about painting on the face, so I normally start the line above the eyeball and end it under it. I take care, though, that on each side of the line, there is a little white area on the eyeball. Then I repeat on the second eye, taking care that the two eyes look at the same direction
Ready