This trick is not hard, it's really easy and fast to execute.
1) First you need to have ready your avatar.
2) With the Eyedropper tool select the color of the skin above the eye of the character in your avy.
3) Create a new layer and with a small and soft brush, cover half of the open eye with the skin color.
4) Create a new transparent layer again and with the same brush, cover the rest of the eye in this new layer.
5) Select "Go to imageready" and simply select the time frames for the layers, obviously the 2 "blink" frames must be faster that the one with the open eye, this to make it more realistic, try to set the "blink" frames to something like 0.1 or 0.2 while leaving the open eye frame in something like 1.0.
Here you go, your avy now makes pretty eyes
Hope it was usefull
