With those polys selected click the "Extrude" button and drag in the viewport to extrude the selected polygons inwards. Select the edges around the hole you just made and chamfer them by 0.3 or so, whatever looks good to you. Test the chamfer by smoothing the geometry to see how it looks.
Well I guess things are moving along nicely now. Now it's time for the fun part! We are going to detach the doors and start thickening out the body shell. Things are going to start moving a little faster now because we are going to be repeating the same techniques that we have been using since the beginning of this tutorial.
Let's add a little more geometry to the door. Cut in the edges shown in green. You can use the "QuickSlice" tool from before to do it quickly. Select the door polygons shown in red and detach them from the main shell. Hide them so we can work on the shell without them getting in the way.
We need to give the door area a lip where the door would rest. You can make this easily with a couple of extrusions. This should be no probelm for you by now. Make the five extrusions shown by the red arrows and adjust vertices to get the right shape.
Zoom in to the bottom corner of the door area and make the small extrusion shown by the red arrows. Cut in the green lines and then weld in the vertices in the blue circle. Create a polygon to close the gap as shown by the yellow lines.
Make the green cut and then delete the resulting red polygon.
Extrude the green edge as shown by the red arrorws and weld the vertices in the green circles.
At the other side of the door, you need to make cuts similar to those you made at the other side. Cut in the green lines.
Extrude the green edges as shown by the red arrows.