Step One: Preparing the Outline
[current materials] Photoshop (v 6.0), obviously. And we are working with 'Rocker Caine', who was generously lent to us by Kelly of the Arcana online shounen ai manga..
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1) Begin.
For those of you who sat through the Cel Shading tutorial, please be patient with me, because I'm going to go through a little of it again. ^_^;; Open the sketch or image you want to airbrush. I've opened up Caine. Feel free to use him for practice, but do not claim his as your own.
Open it up, and immediately cut and paste [Control (Ctrl) 'X', Control (Ctrl) 'V' or Edit>Cut> then Edit>Paste] the image into a new layer. If the Background layer isn't white, get the paintbrush or paintbucket tool and color it white. That's important. If you don't leave it white, any other color will distract you while shading.
Right click on the sketch layer and select 'Layer Properties' to entitle it 'Caine Outline'. This is merely for organizational purposes. Later on, you'll have a ton of layers. *sweatdrops* Bask in the wonders of Photoshop. Giving them names really helps.
2) Clean up the Sketch
You need to clean up the outline now! It's fine the way it is, but making the black outline darker will not only help your image stand out, but it also makes the colors seem bolder as well.
Go to Image>Adjust>Brightness/Contrast.. Leave 0 for the brightness, because the sketch's quality is already very good, but slip in +40 into contract to make the image a lot darker.
It looks darker now, doesn't it? ^^ A lot of artists go over their sketches with the pen tool in Photoshop to ink it. But it saves you SO much more time if you just touch up your image this way.
See, it gets darker here >> 
3) Are we finished editing the sketch yet?!
Wow, you really don't know me, do you? This extra step is needed in case you need to color the sketch outline later to fit the image. Trust me, it seems like you don't need this step now, but it will save you a lot of suffering. ^_~ Unlike the Cel-shading tutorial, I am actually going to show you WHY you'll need this later. Patience, kids! *thwack*
Make sure the outline layer is SELECTED. Then go to the Channels window (Window>Show Channels) and hit the 'select all channels' dotted circle icon [look below]. All of the black areas should be selected. You'll see it, don't worry. ;)
Hit the DELETE key. Yes, delete. The image will fade out a lot. *hears gasp* Don't worry, this is supposed to happen.
Now go back to your Layers menu, make sure the Outline layer is still selected, and click out with your mouse (preferably in the blank grey area), so that the selected areas, those dotted lines, disappear. Control D also deselects the lines.
Look at the Layers menu again, and with the Outline layer selected, select the Lock Transparent Pixels box, as shown in the picture below.
Now select a paintbrush, any size (though I recommend large), and color the sketch BLACK. It should be back to it's nice, clear, clean black color now. ;) Lock transparent pixels means that you can only edit what's already been painted on that layer.
The reason you did this extra step, instead of just multiplying the layer the way SO many other people do (and drive me CRAZY!!!!), is so that you can color your outline later so that it blends in with the picture better. Trust me, it's worth the effort. See, go choose another color, and try coloring it red or blue. And back to black. See what I mean?
NEXT! >> Coloring your Clean Sketch
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