As reference I will use Adriana Lima's eye, because it's a very nice one! Remember to use a detailed and good quality photo as reference.
We start first with drawing the basic shape of the eye. It's very important to get the right shape. You can use a normal pencil for this.
Step 2: Outlines
Let's put in some of the basic lines. It's important to mark the light dot(s) in the pupil/iris. You can also outline some if the black parts that stands out in the reference. I usually use a mechanical pencil for this job.
Step 3: Center of the eye
Remember to fill in enough lines so you can get the shading right. Then you can start filling the center of the eye with graphite. I used 4b or 5b for the darkest, and HB and 2H for the light parts.
Step 4: Upper eye
Now we can draw the upper eyelid and outline the lower one (4B or 5b). Draw the corners a bit darker (HB), and we can also add some dark upper eyelashes. Smudge with a blending tool or a Q-tip.
Step 5: Blending
The eye is beginning to look a bit realistic now, and we can use a
q-tip or a blending tool to achieve more light tones around the eye.
DO NOT smudge the light dot in the pupil!!!
Step 6: More shadows & details
Draw rest of the eyelashes and blend more shadow in. Now we can decide how realistic we want the drawing to be. Add as many details as you want, do all of them if you have the patience. I have added some of them.
Step 7: Eyebrow
Sketch up the eyebrow, and start filling it in.
Step 8: Almost finish
Draw the rest of the eyebrow, and add shadow around the eye. Make sure to get the tone/shadow almost right, and then blend it. You can also add some tiny skin details; this will make it look realistic.