Begin by shading the darker outer tyre first but be careful not to go over the rim which must be much lighter. If you do, just erase it using a clean eraser. Don't be afraid to make the tyre quite dark - the darker the tyre, the more the highlighs on the rim will stand out. When shading the rim, be concious of exactly where the darker areas should be. As you will be working in a confined space, you don't want to be using your eraser too often. Also make sure that you don't forget to draw the brake calipers and discs in (on a sports car with open rims like this, this is very important). Finally, you can draw the finer details such as shading the wheel nuts and the logo into the center of the rim. Don't forget the small holes in the brake discs like I did.