Get Accurate Blueprints
If you don't have accurate blueprints then your car won't be accurate. You also want to keep in mind that the blueprints should be high quality, the car you want, and at least front and side view
The blueprint on the right is pretty good, it has four angles, it's big, has a good amount of detail, and it is the car I want!
Get Reference Photos
Even if you get great blueprints you will still need photos of the real car to replicate the small details, so go on to Google and search for your vehicle, in my case I'll type in, "BMW 1-series coupe" and click, "Images" then look for at least a front-45, a back-45, top side...
Line Up The Blueprints
You don't want to end up half way through modeling the car and your blueprints don't match up, so spend some time getting them lined up using the size, and x and y location.
Start With Key Points
I usually start with the wheel wells or the hood, it makes it easier to begin with a good starting point.
Model in Panels
Since a real car is made in parts and panels, that's what we need to do.
Where to get blueprints