Lorna, it's really easy.
I don't remember if you kept any of the rear heaters, I'm guessing not.
If you want to drain the whole system. There will be a drain plug, or possibly a spigot somewhere on the bottom of the radiator. There are also two
drains on the block, one on each side.
They may or may not have something other than a pipe plug screwed in to the hole. Also remove the lowermost heater hose where it goes into the engine. I recommend that you also flush the cooling system. You can do a pretty good job if you have access to a water hose where you are going to do it. With the heater hose off run water both directions until you get good clean water, make sure you have the heater control valves all open. You can also kind of flush the radiator and block by doing the same thing at the upper radiator hose, but you have to remove the thermostat to do that.
When you are filling, if you don't know the cooling capacity, you can fill using every other gallon antifreeze and water, starting with a gallon of antifreeze. My AARE has a bleed valve on the front heater core accessible when you open the front panel. It was on the upper right and is a 1/8" plastic tube coming out of the top of the piping and going down to behind the bumper with a valve at the end. Run everything until you get good flow and the heater gets hot at both inlet and outlet.
Hope this helps,