I mean just do a solid drawing of the eye, copy the layer, add two layers, and each one, paste the solid drawing there. (Put the ONLY solid drawing and leave it in the bottom layer) The first layer, lower the white (or whatever lighter color) capacity below three and either shade or just dot-dot-dot the solid drawing (TRY USING A PAINTrOLL TOOL) The second layer, do the same with black (or whatever darker color). Lower capacity. Shade or dot-dot-dot. (use paintroll too) Now. Use the eraser tool. Put it at a capacity around 5 or more. Then erase the first top two layers. (Until it looks like a good shade or whatever) And then maybe you're done! It's my way to shade, just so you know.

RLLY SO MY SHADING SMOOTH?