I always rough out my compositions for my images first so I can get a feeling of the final image in my mind before starting any work on the piece. So here in Fig.01, I did my composition study, and then moved on to rendering the girl in the pencil sketch to achieve the 3D form of the figure. I continue to achieve a sense of the pose and structure of the figure, with both lines and shade, until I am ready to scan it in and work digitally.
Note: For this illustration I worked very expressively and rough. I worked very much like a traditional painting as I painted over most of my line and shade work further on in the process.
So I scanned in the pencil shaded sketch at 300 dpi in grayscale, go to image>adjustments>levels and play around with the settings to clean up the sketch a bit, and finally I can start rendering the figure in Photoshop (Fig.02).
First, I set the lines to multiply and painted under the lines to get the general light and shadow down. I grabbed the smudge tool in Photoshop and smudged my pencil lines to smooth them out. When I'm done the general shadows I flattened the line-art layer to my general tones. After I'm done all this, I made a new layer on top of the line art and started painting over the whole figure (Fig.03).
This stage was very important for this piece, as it pretty much decides the whole light source of it. I made sure I suggested the muscles, bones with tones. Also, I wanted to have enough contrast in the figure, lights, mediums, and darks. When I'm done all this, I made a bigger file and canvas, pasted this figure onto it and started to bring in the other elements in pretty much black and white still (Fig.04).
I did the graffiti design here also and worked out every other element. The textures in the edges are all texture brushes effects; you can download these brushes from different artist's blogs or websites, such as the texture brushes Linda Bergkvist (http://www.furiae.com/) Goro Fujita (http://www.area-56.de/) . But some of the brushes I actually used in the piece was created by myself. Look up texture brushes in a search engine for some fast links. So after all the black and white version of your illustration is done. Its time to move on to the colour!
Since I pretty much already have all my tones and shades worked out now in black and white all I need to do now is add colour. Since I have all the tones worked out, I don't need to worry about tone when I add colour. So as you can see in Fig.05, I made a new layer, set the mode of the layer to overlay and painted each part of the figure in one flat colour. Here I used a beige/brownish colour for the skin, a dark blue for her jeans, dark brown for her top, etc.