I remember reading the procedure for a true coolant flush and it was extremely involved. It had to have a computer hooked up to it and ran at a certain RPM for several hours and all sorts of other stuff.
Your best bet is probably to just drain it at the radiator plug. You might be able to also get to one of the coolant plugs on the block. I think the other plug is blocked by the starter. If you can get to both block plugs and drain the radiator you'll have most of it out. I'd probably fill and flush it several times with distilled water if you are switching to a different type of coolant.