Basic dimensions for length, width, height, and most importantly wheelbase are blocked out.

A suitable side-view image is scanned and placed behind the size blocks. Very often the photograph will have been distorted either by the lens used or by variances in the process of printing the image so massaging is necessary. Even the flattest side-view photo is good only as a guide. This photo and the the car's measurements are from World Sportscar Racing '89 by Ulrich Upietz and Gustav Büsing.

And from this photo and measurments, a side-profile is drawn. Some corrections are needed to adjust for any perspective in the photo (moving the cabin, roofline and wing support... anything lying toward the center of the car).