A somewhat simplistic tutorial, but it results in some nice images, nya. ^__^
This tutorial translates well with most color schemes, but reds - especially if they're dark - may need some tweaking of the values in certain adjustment layers.
My usual tips before starting: don't sharpen your base until after following all the steps of the tutorial (you may not even need to), don't do everything on one layer (use adjustment layers, preferably within a layer set), and don't merge your layers (copy merged/paste, instead).
Here's the original scan I nabbed from moe.imouto.org (I downsized, since the original was fricking HUGE). Then a bit of cropping/resizing and we're ready to begin.
01
Let's start things off with a Brightness/Contrast adjustment layer. Brightness: +10 Contrast: +25
Color's better, but the girls are still too bright. So now add a Hue/Saturation adjustment layer. Master: -5, +20, -3
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04
Now we've got a bit more definition and shadow. On to a Channel Mixer adjustment layer. Red: +102, 0, 0, 0 Green: 0, +103, 0, 0 Blue: 0, 0, +103, 0
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05
Now we're bright again, in a good way this time. But... It got worse. Open up a Levels adjustment layer. RGB: 3, 1.04, 245
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06
Ouch. But never fear! Here's where it actually gets better, for good this time. Copy your base and drag it above your other layers at Multiply 25%.
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07
Much nicer, yeah? Now make another base copy and drag it up top, only this time the settings are Luminosity 50%.
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08
*squeaks happily* So pretty~♥ For a bit more clarity, though, let's select all (ctrl+a), copy merged (shft+ctrl+c), and paste (ctrl+v). Now let's use the Sharpen filter, then set the layer to 25% opacity.
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And we're done!
...however, if you so choose to continue, here are a couple of steps to change things up a bit. ^__~
09
Snag a Color Fill adjustment layer with #2C2C2C. Set that to Exclusion 100%.
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10
Finally, let's tack on another Hue/Saturation adjustment layer. Master: +2, +23
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And just go from there to get whatever look you're going for. I'd love to see your results!
Other icons using this technique (from this post):