The starting image is very dark because in this instance only window light was used and no additional lightning was pointed at the girl. So how do we fix this and learn some interesting retouching techniques in between?
First let's start by going to image/adjustment/shadow&highlight and play with the settings until something acceptable comes up. Usually a very small number affecting the amount of dark pixels and a fair tonal amount will do the trick. Additional color correction (on the bottom of the dialog) and midtone contrast patch up a bit of this luminosity deformation.
Alright, let's turn over to the curves dialog (ctrl+m for pcs, cmd+m for macs) and let's tweak the contrast of the image. Firstly make a new curve for the RGB composite values as below:
Now let's tweak the individual channels - red:
Having done that now we can thin-up the girl by deforming her body. Take the marquee tool, drag a selection box around one half of her body.
Now press ctrl+j which will create a new layer based on that selection. What we will do now is bring the part of the body inwards a bit and stretch it in with the free transform tool. To hide the seams on the edges create a layer mask and with a soft brush go around them until satisfactory.
Do the exact same thing with the other part of the body - select, copy the layer, bring in a bit and deform.
By inverting the layer you can see exactly which part the layer is and which part is the original image.
Okay, we're done on this part, now let's go to layer/flatten image and flatten the three layers into one.
We're now fixing the skin blemishes - take the patch tool (selected in the image) and start patching. Make sure the mode is on destination, not source (otherwise it will work inversly) and start selecting the clean parts of the skin on the blemishes. I've highlighted some of the blemishes i found in this screen grab so you have a better idea of what needed to be done.
This just about concludes the tutorial, my friends. The last thing that we have to do is to do a bit of sharpening. Now the amount of it depends on what you want to use the image for (web or print - mine are for print) so you should experiment until the image does not look over or under sharpened.