Materials used: white alkaline paper (letter/A4), a black ball-point pen, a black marker pen, a red marker pen, a red pencil, a blue pencil, a black pencil and a sharpener.
Start with the black ball-point pen, begin with a rough drawing, just a few lines to establish the concept. Reverse the paper and look at it against the light as to check proportion and perspective.
Keep using the ball-point pen. Draw light weight lines to define the body, ground reflected area boundaries, surface transitions and cut lines.
With the same pen, draw medium weight lines to define the body silhouette, window, wheel arch and tire perimeters. This will start to bring the car out. You can also evolve some details of the concept like headlights, mirrors, wheels and define the car ground shadow and the back plate area.
With the black marker pen, fill the back plate, the tires, the car ground shadow, the dark reflected areas on the windows and lamps.
With the red marker pen, fill the ground/dark reflected areas of the body. Use the same color to complete the dark reflection on the windows.
Use the black pencil to get the needed red tone gradation for the body surface transitions. It can also enhance reflection details on the body, wheels and shade plastic/rubber parts like the fascia grille and mirror.
Use the red pencil to represent surface transitions through the tone gradation on the sky/bright reflected areas. These gradients go from red to the paper white. Start smoothly and increase pencil pressure little by little until you are confident with the result. If necessary, practice on a side paper.
With the blue pencil you can represent the sky reflection on the windows, upper body surfaces, wheels and lamp surfaces. Apply smooth gradients again. After doing that, stop for a while and evaluate the sketch from a certain distance in order to get the whole work impression. Go back and enhance or retouch where you think is needed. Use the ball-point and the black marker to reinforce lines, contours, increase contrast/crispness and, therefore, give more life to your work.
Last but not least, use the white pencil to add the highlights , better define the body silhouette against the back plate and, if you want, wash out the grainy aspect of some areas due to pencil use.
The goal here is to show a simple and effective way to professionally represent a design concept. None of the following step images was digitally retouched, but scanned.