I am only introducing this blogger's tutorials because I feel I cannot explain better than he has. He does an amazing job. Please visit his blog and ask him to share more of his content with us:
"Step 1 - Proportions are critical in a technical drawing so I have gridded my outline using 1 inch squares. Gridding consists of placing a grid over your reference picture and on your drawing paper and then transfering the image square by square until you've attained an outline. DO NOT use a hard graphite (ie. 5H, 2H, H) for your grid. If you do you will have a nightmare on your hands later when you are shading. Harder graphites dent the paper, creating tiny grooves. When you go to shade over these grooves, they will not accept graphite and will appear white. Use a softer graphite like a B for your grid. Don't say I didn't warn you :)
Step 2 - As with all of my drawings I am working left to right, top to bottom (just like reading). I am starting on the front of the car and the front wheel. For the wheel I am laying down my darkest tones first with a 7B pencil. These dark tones will give me a reference when deciding how dark to go with everything else on the wheel. The wheels can be very time consuming if you want them to turn out well. They are very small and have lots of details. This one rim alone will take me about an hour.
Step 3 - I add the midtones and lighter tones to the wheel with a 2H for lights and a 3B for mids. These tones are what makes the wheel appear to shine. As with any metal surface, the wheels have brilliant highlights. I always leave these highlights alone so that they are the tone of the paper. So with my midtones and light tones around these brilliant highlights, they appear to shine. This process is called negative drawing and can be explained further by clicking on the link. At this point, the wheel is basically done. When we shade the tires later, these highlights will really start to shine.
Step 4 - With the front wheel finished, I move onto the front fender area. For the darker bottom part of the fender I use a 3B pencil. The lighter top area I am using a 2H. A car finish is typically very smooth so I am blending the body with blending stumps. When working on the body, I am comparing tones to those on the wheel. I'm also erasing grid lines so they don't interfere with my shading.
Step 5 - I go back and work on the tire. Pay close attention to the way the tire looks. There will be some diffuse (duller) highlights or possibly even some brilliant ones if the tire is wet or has tireshine on it. You can see how the rim looks a lot shinier now that we have some darker tones around the negative space. You can make the wheels look as shiny or as dull as you'd like by adjusting the surrounding tones. I will probably go back and darken a lot to make the highlights pop even more. After the tire, I go back to the body with a 3B pencil and blending stumps. You can see how I am just working one square inch at a time. Working this way makes you focus more on details than the drawing as a whole.
Step 6 - I continue working on the body and move up to the window. The tones of everything showing through the window will be slightly muted compared to the rest of the car. So I make the steering wheel and dashboard a little bit lighter. If the window had been opened or tinted this would not be the case though. I used a 2H pencil for the lighter parts of the window, a B pencil for the darker areas and a blending stump to finish it off.
Step 7 - Keep working on the door paying close attention to your reference photo. There are very subtle tone changes in the lighter areas. I'm still using a 2H for lights and a 3B for the darks.
Step 8 - Since our light source is coming from the front of the car, the rearview mirror is reflected off of the body. This reflection will be darker than the mirror itself. Just pay close attention to your reference photo when rendering details like this. After that is done I continue working on the body. The car is starting to take shape now. As mentioned earlier, just take it nice and slow working one area at a time.
Step 9 - I continue working on the body of the Corvette. The top of the car will be a lighter area, considering the light source so I use a 2H on it. I finish the area along the rear wheelwell using a 3B and blending stumps. Then I move onto the middle area of the car, shading with both 2H and B pencils. The doorhandle is one of the darkest area so I use a 7B pencil on it.
Step 10 - I start working on the back windshield. I used a 3B on this area. A shadow underneath the car comes next. The shadow grounds the car and makes it look like it's actually sitting on a surface rather than hovering in space. I use a 3B mechanical pencil on this area pressing fairly hard. Mechanical pencils work better on large dark areas than woodcased pencils. They are sharper and are able to push the graphite deep into the fibres of the paper.
The second card is here: www.vingle.net/posts/254998