On a new layer use the Spot Healing Brush to remove major or distracting veins, but not all detail. Use the same brush to clean distractions from the lashes and around the eye.
Enhance the iris -
The iris is normally lighter away from the direction of light.
Paint with any colour on a new layer over the iris e.g. red. Right click on the layer and bring up Blend If. Hold ALT to bring the bottom shadow to the right to expose the colour only in the highlights. Moving the bottom highlight to the left just exposes the shadows. Change the blend mode to Colour Dodge. Alter the layer colour with Hue/Saturation (CMD + U). Mask out any catchlights showing in the eye. Can change Opacity of the layer to reduce the effect.
Add lightness to the middle area of the iris by painting white on a new layer above, using Blend IF on Soft Light and reducing the opacity. This emphasises the natural patterning in the eyes.
Then Dodge and Burn the iris to make the inner and outer areas a little darker and the middle area lighter. Leave a darker shadow on the top of the eyeball under the brow. Can also darken the pupil.
Examples of adding more colour that I have not noted.
Dodge and Burn to emphasise the lashes and socket. Darken lashes just like makeup does. Lighten just inside to add contrast. Emphasise brow by darkening above the eye and lightening the brow. Also lighten above the eyebrow line.
Sharpen the eye. New Layer. Image > Apply Image. Use unsharp Mask and mask this to just reveal the eye but just the iris and eyelashes, possibly eyebrows. Lower opacity if needed.
Note, Phlearn didn't lighten the whites of the eyes routinely.
See Phlearn Photoshop 201, no 8 - 47.00-48.40. Create a new layer. Image > Apply Image ensuring merged layers is selected. Blend mode multiply or normal. Convert it to a smart object. Filter > Other > High Pass and choose a radius. Choose a blend mode of vivid light. Add a black layer mask and paint in the eyes in white. Can also do the eyelashes and lips but avoid any skin.
Emphasise Eye Colour
See Phlearn Photoshop 201, no 8 - 47.40-50.30. On a new layer paint with an emphasis colour e.g. light blue using a small brush to paint flecks radiating out from the iris. Change blend mode to colour dodge. Use hue/saturation. Can dodge to darken the outer circle of the iris and burn to lighten the inner circle.
Remove the bags or dark tones under the eyes but don't remove the shadow under the lower lid as this makes the eyes look unnatural.
Techniques - use the healing brush on another layer, sample good skin then extend to the bags but do it all in one, then let go at the end.
Use the Patch tool and do a rough selection and replace, which removes it completely. While it is still highlighted click the Edit button in the menu and there is now a Fade button to fade the selection. Use something like 50% to edit this more realistically. (Creative Live - Home Studio, video 30).
Dani Diamond - 01, 02, - A more sophisticated way. 1. Add a Curve Layer 2. Click on the left side of the Curve Layer (this will place a white box around the curve icon) 3. **Double Click* on the Top Eyedropper (Sample Black Point) This is key for my Photoshop CC 2015 setup) 4. Color Picker will come up at this point. 4. Sample Light Color 5. Close Color Picker by clicking "OK" 6. New Message Box pop-up asking to save target color Click "No" 7. Sample Dark Color 8. Select White Mask and use Control I to invert mask from white to black. 9. Select Brush tool and set Opacity to a small number and then start painting.
Thicken Eyebrows. Phlearn. Use the pen tool to make a selection and fether it by one pixel, Use a large, soft edge brush on another layer, pick the eyebrow colour. Use Hide the selection CMD + H. Multiply blend mode. Use a low flow and paint in the colour subtly so that it is darker in the centre. Blur the edges. - Add the hairs on a new layer using Multiply. Can make a brush to fade the length by altering Shape Dynamics under Jitter by adjusting Control to Fade and adding a number for length (Pixels?). Using a tablet alter the Control to Pressure Sensitive.
Phlearn - To make the eyes look bigger make the inside eye shadow lighter and the outside one darker. *** more to note - principles of eye makeup and realism, and eyelash brushes ***
Catchlights can be edited or added to the eyes as without them the eyes can look dead and flat. (Ref)