You should give the eyeball a spherical and large appearance. At its front would be an egg shaped protruding bulge. This is where the lens would be located. Atop the eyeball, you can find the eyelids. It is stretched over the top potion of the eyeball.
In drawing, the most important part of the eyeball would be the white portion of the eye. In reality, these do not really have a pure white color. They also have shadows in them. So, remember that the shape you are aiming for would be a sphere. So, try to make a sphere that has some shading for it to become a realistic eyeball.
For the eyelids, it was said that it is stretched above the eyeball. Thus, the eyeball curvature would be the one to give the lids their shape. You should understand and memorize this factor. It is also vital that you exactly capture the eyelid line shape. Observe that they generally follow the eyeball curvature. Take note that every time eyelids are retracted, they have a clearly visible crease that should be accurately rendered.
Lastly, see the odd little shape situated on the inside corner of your eye where the lower and upper eyelids join. Make sure that you draw this shape as how you have had seen it with your subject.
Make sure that eyelashes are sprouting out from the eyelids and not anywhere else from the eye. These usually grow in clumps. Thus, draw them that way. Spend some time how they grow and the direction they are going. Take note that there is a difference in size for eyelashes at the bottom lid compared to the upper lid. The ones on the upper lid are larger. Since they come from a clump, they are also usually entangled.