My renderings are usually no larger than 750 pixels wide. If you're working with anything considerably larger then your results will differ slightly.
Scan your drawing of the car. Don't worry if the lines are a bit rough we will be drawing them again in Photoshop.
When you first open the image in Photoshop it will be on a layer named Background. Double click on Background in the Layer Palette and a New Layer dialog box will appear. Call the layer Original and click ok.
Rotate the image (Edit > Free Transform, or ctrl + T), to give it a nice angle.
Use the Pen Tool, (P), to draw over the original lines, and the Direct Selection Tool, (S), to refine the paths.
(Notice the pen tool has a small black triangle/arrow in the bottom right hand corner, this means that there are additional tools hidden in that tool. To access these tools click and hold the left mouse button. The Pen Tool has the following additional tools.
Use the bottom three to refine your paths. Add Anchor Point Tool and Delete Anchor Point Tool are fairly obvious and the Convert Point Tool lets you to redefine the anchor points and give a smooth corner).
Select the Brush Tool, (B), right click over the image and select brush of thickness 1.Make sure your Layers palette is visible (F7, or Windows > Layers), make a new layer and call it Outline.
Select the Pen Tool, (P), again, right click and select stroke path and select brush from the dialog box.
Now click esc twice to hide the paths.
Make a new layer and call it Shadows.
Make sure that the foreground colour is black or whatever colour you want for the shadows.
Using the Wand Tool, (W), Select the tyres and the shadow areas underneath the vehicle. Select the Rectangular Marquee Tool, (M), right click inside the selected areas and choose Fill... from the list of options. Make sure foreground colour is selected and click ok.
Using the tools described above add a shadow area at the bottom of the vehicle. Create an area at the bottom of the car to represent the atmosphere.