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.
If you're looking to add a light to an existing ceiling fan, making it a combination fan/light, this will only work if (1) the fan is designed to accept an optional light add-on and (2) you use the light add-on appropriate to your fan. If you don't know what type of fan you have, this will be difficult to find, but you could start by turning off the power and seeing if you can detach a part on the bottom of the fan. If this reveals some wires or wiring terminals you may be in luck; measure any attachment points and look for an add-on from the same manufacturer that would fit.
If you're talking about just swapping out the whole fixture, you could relatively easily replace the existing ceiling fans or light fixtures with new fans or fixtures. Most fixtures are designed to attach to standard ceiling boxes. These sorts of changes can also be easily undone (remove your new fixture and reattach the old one) if needed, which may be useful in your situation. Note that if you're attaching a ceiling fan, you need to be sure the box you're attaching to is rated for fan use (i.e. has a stronger attachment to framing to support the weight/motion of the fan).
Best Answer
Here is a link to lots of images of a variety of surface wiring hardware. There are several manufacturers of good looking surface boxes and raceways. One of the popular high quality makers is Wiremold.