Try to identify the main contour shapes on the car as well as the main panels (doors, windshield, headlights etc.): I usually start from the side view, then front and top, check everything, add another spline and so on. When creating splines, try to already figure out where to put vertices (ie. where 2 splines cross, you should have a vertex on both, and possibly using snap make sure they are coincident). You can always move/add/delete vertices on a later stage anyway. Also, model just one side of the car, we will mirror it later. It's a good idea to use grid snap to align the middle vertices too. Use Fig. 2 and Fig. 3 as a reference to see how I started laying the splines in the 3d space.

Once you have the main lines in place, it's time to add some details and "volume", and to refine vertices positions. In Fig. 4 you can see what I mean: start taking a look at your reference photos to see where a car panel bends or if there are important lines you couldn't notice on the blueprints. Then start adding lines (I mainly use 2 points splines in this part) and start forming a "quad cage" (quad means you should try to have a 4 sided cage wherever possible): you should try to imagine where polygons will be, and they will need to be 3 or 4 sided later when we apply the "surface" modifier.

fig.3: drawing splines check that
the perpective view makes sense

fig.4: adding more splines start adding detail and "volume" splines

fig.2: last splining steps try to keep all quads and check vertices positions on the various intersections

TIn all this process you don't need to be 100% precise with the blueprints, all the vertices you are placing will be more or less moved on a later stage, believe me.

The next part is quite slow but fairly easy: keep adding splines untill you get a reasonably dense cage, trying to get splines where you think they will be needed (for example, noting the depressed area of the front bonnet in a photo, I added a spline a bit lower than the contour spline). Don't be afraid to delete a spline you don't like. Notice the mistake I made here (Fig. 5): the bright red circle shows an area where I laid the splines badly: the bright red line shows where I should have put a spline which is missing at this stage of the modeling, also in the circle there are 3 usless splines, the purple, the green and the blue ones. They do not identify any particular geometry, moreover they somewhat "break" the regularity of the cage. God knows what I had on my mind. Also the headlight spline is not fitting properly in the cage: I will not use it in the surfacing phase, so it can be deleted: there's already a quad in that area, which later will be easily adjusted to make room for the headlight. Once you feel you have enough splines to describe the main car geometry, you are ready for the next section.