How do I install this flush mount ceiling light fixture?
Before you start, make sure the power to the fixture is off (not just at the switch) and, if possible, confirm with a non-contact tester (a few bucks at a hardware store).

D are the screws that need to be put through B, which is the bracket that will be attached to the ceiling box to hold up the fixture. 

To see which holes, align the bracket with the holes in the fixture base. Thread screws D through the back of the bracket so the threaded ends stick out front. Which side of the bracket is the back depends on the ceiling box and whether you need the offset on the bracket sticking down or up. (It often doesn't matter.)

C are the screws that hold the bracket B to the ceiling box. Once the bracket is installed, you can connect the wires on the fixture to the wires in the ceiling box (including the ground wire to the green screw on the bracket).

Place the fixture against the ceiling with the threaded ends of D sticking through the holes. Attach with the nuts E.

The threaded tube on A is screwed into the center of the fixture, the nut is then tightened down the tube to the fixture.  The remaining parts are removed. After bulbs are installed, the globe is put on, followed by the soft washer, the hard washer and the next nut. Don't overtighten or you can crack the globe. Then the decorative cap is put on and then the ball finial.