Learn how to draw a Porsche 911 GT3 (997). You should be able to do it in around 5 minutes once you understand the proportions.
The wheelbase is 2.5 wheels, the front overhang is 3/4 of a wheel long and the and rear overhang is around 3/4 of a wheel long as well.
The greenhouse is a little over half a wheel tall.
Take a look at how the relevant points that dictate the car's proportions (where the windshield starts, where the A pillar starts, where both meet, where the trunk starts and ends, etc) are placed relative to the wheels drawn at the beginning. It's all about joining those points correctly, with either straight, concave or convex lines.