Select the polys and inset as shown. Extrude them outwards by about 1. Now just chamfer edges and add symmetry and meshsmooth modifiers.
Place it in the little cavity that we made in the door area of the car body and shape it to fit in the spot so it's not as square.
Make the latch on the actual door itself by just modifying a cylinder as shown. This is not very realstic and you can make it more realistic if you like but for the purposes of this tutorial, we'll stick with this.
We might as well put the side mirrors on the doors. Start with a box with 3 length segs, 4 width segs and 2 height segs. Shape it into the shape shown in the picture.