The modeling of the body of a car can almost always be sliced up into different parts, literally and figuratively speaking. Litereally because most of the time the front and rear bumpers, the entire 'cockpit', mirrors etc. are all loose parts. Figuratively because when you model it, it can be done in steps: first a rough simple (but rightly shaped!) basis and then the refinements, such as chamfering edges and making the creases.
Setting up blueprints is a 5 minute job, I have explained the way I do it earlier in the tutorial about how to setup blueprints. Only this time I chose a different car, since almost the whole world seems to model it: the Audi TT. Another reason to model this car, is because it's one of the easiest car to model. After we have done that (1), it's time to jump in.
PS: When I would be modeling a car I would try to find as much reference photo's as possible. As I have stated before: I don't use the blueprints that much when I model my car, only for the rough / global shape of the car I use them, photo's are way better reference since they show all details. So get Google out and try to get your hands on wallpapers and personal photo's of the car you try to model. Sometimes it can be very hard to find any (for instance for the 993 GT2 and the Alfa Romeo SZ I modeled), but it's essential if you want to end up with a good representation of the original. I am not talking about a couple of photo's... usually I would try to find around 50-100 wallpaper sized (800x600 or larger) photo's. In this tutorial I won't go into modeling all details, since only the concept is important, not the modeling itself.
Finding a Place to Start
I have the feeling that many people have a (healthy!) fear or starting to model a car. 'Where do I start?', is a logical question. And there is no definate answer. Anywhere would be the best I can think of. It just doesn't matter, yet the way you approach the modeling process does.
So I am just going to start at the front, and model the hood. What I will think of from the first second I start is that the whole car is symmetric (apart from the tank cap), so I am not going to model both sides, but only one half. So I'll start with a single plane, but the way I create it, is by creating a box from the side view (1) and then deleting 5 out of 6 polygons (2). This way side of the polygon is exactly in the middle and I can use the symmetry modifier without any troubles (3).
So now that we have made the basis, the hard work is up! I start from the top view and shift-drag some edges, slice once and move the vertices to match up with the blueprints (but only from the top view up until now!) to create the entire hood (1). After doing that I realize that the most front part of the hood goes downward and that I need more polygons there (what you see is smaller than it really is, because of the orthogonal top view) and so I model the front grill too (2). I always model this with the symmetry modifier visible, so I can see the result immediately.
Now is as good as any time to not only make it match the top but also from the side (top and side view are most important, I think, front and rear view I don't use very often). So we move the vertices so they match. Both the most inner row and outer row of vertices are easy to place (3), since the blueprints have a line there (the crease of the hood) in the top and side view. The line of vertices in between should be placed by feeling...
PS: Notice how the blueprints I used do not match up. This is a common flaw of almost any blueprint, and there is nothing you can do about it, just live with it and correct your modeling so that it looks best according to your own feeling. This is also why I say that you shouldn't trust on blueprints alone.
After making the two other rows match too, you shold have something like (1). Looking good if you ask me. Realize that the lines on blueprints usually correspond to creases and / or sharp edges. When you want to model them, you need edges there (to create the creases as explained in a previous tutorial).
Well we can't go any more further to the front, all is done there (well roughly), so we should go towards the side. I selected the entire row of edges (2), shift-dragged them out (3) (once again) and editted the vertices I just created to match up again from top and side (4). Looks fairly good, but I made a small mistake: in the last part I didn't realize I was looking from above again and so I didn't have enough vertices to match the shape of the body, this I will set right by cutting an extra row and place the newly created vertices correctly (5).
Since I won't be modeling the entire car (why would I? Then the tutorial would cover how to model an Audi TT ;), I will now start with some detailing (which I would normally only do until I have finished the rough model of the entire body.
The edges on the hood are a good place to start. First we select the edges (1), chamfer them (2), move one lower than the other (3) then chamfer the new edges again only now smaller so they will become more sharp (4). Add a MS modifier with 2 iteration to see whether the result is satisfactory.
I now will model the headlights and the crease of the hood. Since the headlights look to be perfectly smooth with the body I will just make creases around it (and not delete them and later on try to remodel them back in). And with one sweep I will also create the crease of the hood. So I select the edges (1) I want to crease and apply the technique explained in an earlier tutorial (2). With a 2 iterations MS modifier (3).
Would I continue from here, then I would be in deep trouble, making the creases continue onto the wheel arches is undoable (as explained). So realize that this is just for tutorial purposes that I start detailing already.
Last thing I will do is make the grill. I didn't take into account modeling it (I have no vertices there), so I will slice where needed (1), then extrude inwards (2), delete the not need polygons (3) and finally chamfer some edges (4).
So in the end it looks like this:
Modeling the rest of the body is just more of the same. Only you would first model the entire car, then make the creases and hard edges and then do all the final tweaking.