Now this is a bit off a jump but I think you should be able to get it. Create the cut shown in blue then select the edges shown in green and extrude them inwards in the direction of the red arrow a little distance. Create the polys shown in red with the create too we used before. You don't really have to worry too much about this area since it will be obscured by the door. However, if you are ever going to render the car with the door open, it's a good idea to model this part since it will be visible.
Now all you have to do is fill in the hole. This should be easy enough and you can do it pretty much any way you want. The way I did it was to cut in the lines in blue and then make the polys shown in red. Do it any way you see fit. As usual, make sure you tweak the vertices to get a nice shape.
Now let's start into the car. Extrude the green edge a number of times as shown by the red arrow. Make sure the vertices at the center have an x-coordinate of zero to ensure that they are perfectly centered.
Extrude the green edges inwards a little to create the polys colored in red.
Select the polys that you made to the center of the car as shown (red) and detach them from the rest of the car. Disable the meshsmooth on the rest of the car so that it stays low poly. Select the detached polys.
From the inside of the car, extrude the green edges in the directions shown by the red arrows.