Fill your background layer with the color #c99e7e. Then add a new layer and draw a basic lineart with a dark red/brown color. Don’t be too picky, as the lineart will be covered later.
Make a new layer between your background and lineart. Using a big soft edged airbrush block in the basic shadows and lights. Use the colors next to the panel. We are going to use brown/red for the skin-shadows and yellow for the lights. The eye is usually a mix of greenfish blue and brown/red. The eyebrow is brown, but you can make it any color you want. User references if you don’t know where to place the colors (I do that a lot too).
Merge the background and the light/shadow-layer into one layer (not necessary, but I hate having too many layers) and make a new one above. Now, still using the soft airbrush, block in the color for the iris. I want to draw blue/greenfish eyes. As the basic color choose a rather dark, not too saturated color. The glow of the eyes will be added later.
On the same layer draw the basic shadow, using a dark saturated blue. The iris is not flat, it is usually really dark in the upper part and bright in the lower part. Also draw the pupil, using the same brush and color as well as a dark edging around the iris.
Now we come to the lightening of the iris. Draw little ovals around the iris using a bright-saturated blue (cyan-blue). This is the main structure of our fantasy eyes. We are going to add structure and more effects to it later.
Still on the same layer, draw the little light-dot next to the pupil, using white or a very bright cyan. I drew two of them to give the eye more depth. Also I lowered the opacity of my lineart layer from 100% to about 60% as we are going to cover the lineart in the next step.
Now comes a very important step. We have the basic structure and composition, so we are going to draw over the lineart. In this step we use the hard-edged airbrush and the color picker to get smooth transitions. Make a new layer above your lineart layer (so on top of all your layers). Start drawing gently with smooth movements. Also begin to draw first details in the eye and eyebrow (again, don’t be afraid of using references, every artists started using them).
On the same layer, still using the hard-edged airbrush we begin to draw details on the skin. Human skinhas many surface irregularities and pores as well as folds. I also drew little veins into the eye using a very small brush size (2-3px). In this step we also refine the eyebrows and draw the eyelashes, using a small hard airbrush.
We are nearly done! Make a new layer above all your layers and draw little details and structures into the iris. Use a small hard-edged airbrush and the color picker. Draw small highlights and also choose color variations, such as green, cyan or even purple (everything is possible). Give your creativity full scope, you can even draw little symbols into your eye.
Now comes the last important step. Make a new layer above all your layers and set it to “overlay”. Now use a big soft airbrush and a light cyan (or any other color) to make your eye glow. But don’t exaggerate it! Using the same brush get some color variations into the skin, e.g. some purple and blue into the shades or orange and yellow into the lights. But do this really gently! Draw the last little details and highlights and make some color/light/contrast corrections. Then choose Filter > Sharpen to give your picture the finishing touch.
Here are some other useful tips on how to draw various eyes:
If you would like to continue, you can learn how to draw fantasy lips here.