STEP 1: Grab a pencil, three pens ( I use Pilot 4.0 pens) of a different colour each ( i use black, red, blue).
STEP 2: Grab (i dont know exactly how its called) graph papper/millimeter papper, it makes calculations A LOT EASIER.
STEP 3: (VERY IMPORTANT), turn on, as loud as the circumstances allow you, music, preferably metal, rock, or hard rock, making cars while listening to this kind of music gives a lot of inspiration ( especially with trucks, hotrods,monster trucks).
STEP 4: Read the tutorial at the official site ( you can folllow the link to it trough the car maker).
STEP 5: Draw your car, in 2D with the "x" values already defined on papper, DO NOT mark the piece's colours on the papper. Use different colours for different parts of your car will be less confusing reading it later.
ex.: Black for main body, Blue for windows and lights,Red for supercharger, etc
STEP 6: Make the polys in the car maker, and mirror them alongside "x" axis. Divide the polys into bunches and mark each one with a word or sentence, something like "SUPERCHARGER, SIDES, FRONT LIGHTS, REAR PART, FRONT PART" ,etc. It helps a LOT if you have to find your polygons later.
STEP 7: If your making a similar car to your previous one use parts of it on your new car, with a bit of modding them and recoloring tthey will look very different (im not talking about using the whole side of your car!!)
STEP 8: Add your desired stats, wheels and testdrive your car.