I don't drain anything. You won't loose much, the pump is pretty high in the system. Loosen the pulley bolts before taking the belt off. A quick snap of the wrench will break them loose. The belt will usually keep the pulley from turning just enough to break them. If not, you can push on the top of the belt to tighten it's grip a little, and that'll usually do it.
Replacing the pump is very simple, with the pulley off it is totally self-explanatory. Make sure the gasket surfaces are clean and you torque the bolts evenly. Don't get real aggressive with a scraper, that casting is aluminum and will scratch/chunk easy.
The most critical part of the job is getting the system bled correctly after you are done. That's where it goes wrong sometimes. You'll want to open the bleeder screws (one above the pump at the top of the pipe, the other near the thermostat) and fill the radiator until coolant is coming from both.
Then close the bleeders, cap the radiator and start it up. Let it run about fifteen minutes while you watch the temp gauge. Have the heater control on full heat too (just the temp selector, not the blower). Once it is showing a normal temp on the gauge shut the motor off, let it sit about ten minutes and carefully remove the radiator cap.
Open the bleeders once again and top off the system until coolant flows from them. Cap the radiator and go for a drive, watching the gauge. If it spikes high or the heat from the heater isn't consistant, you've still got air in there. Bleed it until it doesn't have any more problems.
Most of these bleed pretty easily. Usually they are fine after the first top-off after running it.