Sorry, there's no real step-by-step guide, I forgot to make steps in between, but later on I added some more stuff so I can provide at least a few different pictures.

So this is the original

I used the "Patch Tool" and "Clone Stamp Tool" to get rid of the pimples, a couple of wrinkles and other unpretty things. With the "Dodge-" and "Burn-Tool" I added some light and shadows on the skin to let it look less flat. With various blending modes and image-adjustments such as Contrast, Hue/Saturation, Curves and some other very little fixes all the time to get rid of very small things (i can't count it all up), I tried to give it a better outcome. I wanted to keep the same lightsettings, so I didn't make big changes on the contrast etc. With the Liquify-Tool, I changed the noses and the teeth and other small things like the chin.

That already was my finished entry, but then I changed some more:
I tried to make the lips sharper and clearer to see, before it was quite pixelish and had unsharp edges, now they are a bit sharper.
I fixed the skin between chin and mouth, same on the forehead and a bit on the eyebrows, now the shadow and lights on the skin should look better to give it a less flat feeling.

This is just a very little step actually, I added some shadows and lights on the skin (left side of forehead and right cheek), as before, with the Dodge- and Burn-Tool and the Clone Stamp Tool.

So then everything was finished, here's a side-by-side comparison.

The last step I did was to cut out the girl (thanks again, akasha) and show the original and the final version of her next to each other.