A tutorial by Miroslav Dimitrov, author of the Dacia Logan 2012 concept:
I have used one of my Berman Design Competition renderings for this tutorial. Although I was allowed to modify only some parts of the vehicle, I started the render from the scratch. Another thing is that I am not a fan of the underlays at all, I use them only for specific jobs. However, for this render alone I used an underlay of the original concept car because I wanted to be as precise as possible. In other words, to apply my design I had to keep close to the original concept car.
Let’s start with the tutorial.

1 - The first step – prepare the render for the shadings. In my case, I already had the design finalized so I just had to trace the original vehicle and adjust where my design alters from the Renault Koleos concept.
In Photoshop I made a new layer, containing the original picture of the car. I turned down the opacity of the layer to make it less visible.
Now it’s time to do the outlines of our design. As already mentioned I wanted to be very precise & clean, which is the reason why I used the path tool in Photoshop, to stroke the lines in another layer. The rest of the outline work was done by hand with the help of the Wacom tablet.

2 - Before carry on, I turned the visibility of the traced image down to zero.
You got to get used to create a lot of layers in the Photoshop. This will help you organize your work, it is also very helpful If you want to change something at the later stage of your render.
New layer – this is the base colour of the car, we will build all the shadows and highlights on top of this one. I used proper grey colour for that. I made another layer with 100% black colour for the tyres and the shadows under the vehicle. Notice the seats and the inside of the car. This is another layer filled with darker grey colour. Make sure you keep your drawing clean and not leave any un-erased parts left over the image.

3 - It is time to build the main volumes of the car. You must consider the direction of the light source too.
Remember, always the big volumes first! Create the big volumes with the big brush and play with the opacity of the layers till you reach the desired density of the shadows and the highlights too. We have to be conscious of our outline work underneath in order to deliver the design we had in mind.

4 - We continue building the render. More shadows and highlights are added to the body of the car. Bare in mind that the shadow follows the shape always. The better we spray our shadows over the render, the better they will represent the volumes and the surfaces of the design we’re trying to show.
A tip – If you can’t accomplish the desired shape of a certain shadow/highlight, use the erase tool in Photoshop, until you achieve the shape you’re happy with.

5 - As can be seen, I have decided to use the original wheels of the Renault Koleos Concept – very simple trick, paste them in a new layer and manipulate till you find the right place for them.
It is time for more details and refined study of the shapes & volumes. This is one of the sweetest parts, I like it very much because here I can play around till I reach the desired effect. It also brings some maturity to the render. For instance – the wheel arches.
I carried on with the door handles, side skirts, windows etc.

6 - Before we reach the final stage, we have some more things to do. We must finish with our shadings and complete all the details such as fenders, headlights, trims, skirts, mirrors and so on.
A tip – when you’re happy with your work, merge several layers together to make one layer with a certain finished piece of your rendering. For example – the fenders. This will make your life much easier when scrolling through the layers list. It will save some resources of your computer and it’s much easier to handle in Photoshop.
Now the shut lines and the grooves. I use a thin brush for the shut lines or stroke a path for some continuous complicated shapes only if really needed.
After we’re finished, we can now flatten the image into one single layer.

Before presenting It, I filled the background with a gradient to make the car itself stand-out and bring some deepness to the whole render too.
The whole rendering took me 3 hours and something.