The big thing is that if your aperture ring has an "A" setting on it, put it there and leave it there. That's easiest.
However, I believe the "M" series lenses don't have that. So the procedure is to set the camera to Manual mode. Set the aperture about where you think you'll want it. (The lower the number, the less light you'll need, but depth of field shrinks so focus is more critical, and at the max, sharpness can decrease, but I think that lens should still be very sharp at F2.0.) Hit the AE-L button. This will "stop down" the lens and take a meter reading, and set the shutter speed appropriately. You can now fire away and should be accurate exposure. If the light changes, you will want to press the AE-L button again to perform the stop-down metering. If you think that the resulting photo is too bright or too dark, you can adjust the aperture via the ring on the lens, or the shutter speed via the dial on the back of the camera.
It's an extra step but pretty painless when you get used to it. The only time it's really a bother is if you are in quickly-changing light conditions - I used a lens that required this on Splash Mountain, and didn't really have time to meter properly going from inside to the big splash (as you only have an instant or two between looking at the bright sky, and going down the big drop where I was trying to take some shots.) On the other hands, the same lens worked wonderfully on Big Thunder.
