This is the english version of my step-by-step tutorial (here) for the non-french speakers around here. Hope my english's not too broken !
Also, basic knowledge of photoshop is required ! (you have to at least know how layer masks work)

This is the basic part of the drawing. I start by making a rough just to see the general composition. Well most of the time (this one included), I actually make up the “general composition” as I go. Yeah, I know.

Then it’s time to refine the lineart to make it look, well, like lines.

Then it’s time for the final lineart (beside some last minute modifications). This is one of the most time consuming step, and not the funniest, so I often think about skipping it and getting on with the colours from a rougher lineart. However, it seems I can’t do that. Deep-rooted habitudes anyone?

I make a lot of layers. For everything. Yes those layers are of lineart only. It really lengthens the process because you’re always juggling with layers, but it allows me to colour my lineart much, much quicker towards the end of the drawing. However, I can’t really tell if it’s really faster this way.

Follows the second long-and-not-very-exhilarating phase: flat colours. While it’s very quick for a simple drawing, it becomes a lot longer as the drawing gets more complicated. Well whatever, I guess everything takes me a long time anyway. Oh and this is usually the phase when you tell yourself: “wow, this looks like crap!” But keep going! I keep all the colours on separate layers. Again, more confusion but more control. At least I have to keep separated the layers which will receive textures.

Ok it’s time to shade this baby! (I’m talking about the drawing, not the girl). Again, here’s my time consuming method:
Make a folder, put it in “multiply” mode, and then put your shadow layers in it in “normal” mode, from the lightest to the darkest (the darkest on top, as seen in the caps). Why using this multiply mode? Well it allows me to keep the shading consistent throughout the whole drawing. Oh yeah, it takes more time too.
Then, over the shading folder, use a normal layer to do your precise shadow works and all the “abnormal” shadows (like colored or even darker shadows). It’s very handy to keep a palette with the different shading tone you use (you can see mine on some screen caps)

Then it the exact mirror process for lighting (use the “overlay” mode). Don’t forget the specular lights!

For the metal parts, it’s a bit different, since the specular factor is very high.

Then generalize this for the whole drawing. Again, I keep adding layers. I guess I could do with just one set of shading layers, but it allows me to keep better control on the later stages of the process. And I like control. Oh yes.

So now the fun begins! Texture time! Import one of the many skin texture files you must have! What? I made mine by merging a lot of skins I found on high-res stock photos (yeah, nude ones mostly) and my own skin. Yes, I sound like Frankenstein here.

The major problem with this texture is that it covers a small area of skin, like 30x20 centimeters. So I start resizing it down, duplicate it a lot, making sure the orientation is never the same and the most “visible” spots on the texture are not repeated too often.

I then use the wrap tool to, err, wrap it around the body parts. This is luxury as the effect is almost imperceptible, but it’s fun to do!

Remember that skin color layer I kept separated? It comes in handy now! Well actually I don’t need the layer, just its selection (control+click on the layer). Make a new folder and then use this selection as a layer mask (in the later versions, just creating a layer mask when a selection I active will make this selection as a mask.

Then play with the levels.

Then, Photoshop magic !
In the texture folder, 3 layers, all duplicates of the texture, in order :
An “overlay layer” with 5-10% opacity
A “multiply” layer with the desaturated texture! Opacity 10-20%
A “multiply” layer with 30-60% opacity.

But hey it’s CERTAINLY not enough skin texture! So choose one of your many freckles textures, and repeat the previous steps!

For the floorboard texture, it’s almost the same, but instead of warping it, you have to use the “perspective” transformation.

Beware, though: adding all those textures will make your filesize explode! At this point of the drawing, my file size skyrocketed up to 250 MB, which makes everything go slower. So don’t hesitate to merge layers from time to time!

A crucial step for me who works with lineart is to make this lineart disappear, partly or totally. Well no techniques here, just brushing on the lines to color them. Of course having kept all the lineart layers separate makes this step easy!

Now it’s time for some backlights! It’s like lights, from the back… err I mean lighting that comes from whether another light source, or a reflection. Well, above all, this is the step where I put shiny colored lights wherever I want, to make it “look good”, but shh, don’t tell anyone.

At last, we’re nearing the end! When two objects touch, there’s usually a shaded part where the two objects come in contact, because they’re blocking each other lights, the little egoistst! So in order to make objects “stick together”, I add a layer of shadows were they touch. (Take notice of the front seat, for example).

Lastly, it’s time to CHEAT! Yeah it’s a tutorial about cheating with Photoshop to make instantaneous good drawings! CHEAT WITH PHOTOSHOP! WIN A FREE IPOD!
So what do we have here? The drawing is finished, but it lacks ambiance, and the foreground and background are almost undistinguishable! And I’ve spent twenty fucking hours on it! Daaaaamn! But worry not, because your saviour is here, and its name is “gradient map”.
All I need is to keep a selection of the foreground (for example, the character and her seat), in one form or another.
Flatten the image
Duplicate it and use the gradient map tool. For those who don’t know, it’s a tool that will replace all the drawing colours by a specific gradient, only based on the value of the existing colour. Yeah it’s hard to explain with words, just try it and you’ll understand

Create the gradient corresponding to the desired ambiance.

Save it by clicking on “new” (and not “save”, go figure)
Duplicate the original image again and use the gradient map tool again. Get your old gradient by clicking on it in the palette, but this time, make it darker!

Put the lighter version layer on top. And put the foreground selection as a layer mask on it.

Merge the two layers. You have your ambiance colors!

Okay here’s a difficult one. As I see it, the more in the background something is, the more the “ambiance colors” should predominate on its natural colors. So make a new layer mask like this (the “ambiance colors” are less visible on the foreground).

Finally make your layer’s opacity 30-70% depending on your tastes.

And while you still have your selection available, merge everything, duplicate it, blur the top layer and make a layer mask so that only the background is blurry.

Aaaaand you’re done.
Easy isn’t it?

(real time spent between the beginning and the end: more than 3 weeks, ha ha ha)

Extra stage! How to add this semi-transparent top you planned from the beginning but totally-crap-crap-crap forgot about?

On Top of everything: Lineart, flat colors, shading, lighting and lineart coloring, okay? Well you have to remove the part that is hidden by the hair. The surface is not complicated so you can do it by hand, or else, if you still have an old layer somewhere with the hair selection, you can use it as layer mask. Also don’t spend much time to polish the shading, as most of it will be transparent.

Then make a blank layer mask, and select a nice looking brush. Here I took a flower one. With the brush options, choose “scatter” and “shape dynamics>angle jitter”, choose the “multiply” mode (for the brush, not the layer!), a grey color, and then go wild!

Now you, like me, have spent way too much time on this, so quick! Close this PSD file and never reopen it again! If by any chance you're an english-speaking reader who doesn't come from Deviant art, here's the final result :