The .PDF instructions 2kg4u gave are almost exactly good. There'* a slight difference in disconnecting the heater position motor but all the rest is very good.
Just be careful when you're going to put the new heater back. It'* a hassle to get out, it doesn't get better when going back in and you don want to crush the cooling fins.
Quick pro-tip :
GET A COOLANT CLAMP REMOTE REMOVAL TOOL!!! The supercharged Bonneville'* heater hoses are pretty much unreachable with any tool except that. 30$ on Ebay.
-Dismantle passenger kick panel
-Spot the "vacuum controller?" It has colored tubes going to it and a connector. Disconnect tubes (remove central screw and entire connector, don't remove tube by tube) and wires, remove controller.
-Remove glove box door. Doing so will allow you to fiddle with a long and thin screwdriver to unscrew one of the heater door screws. That'* the part where the .PDF instructions are not exact.
-Remove the two heater box screws (Maybe more, but I think I remember two (you don't forget a shitty job so easily!!))
-Remove heater core
-Put new heater core in using extreme care
-You may want to silicon the heater assembly parts before putting them back in
Installation is reverse operation
Good luck. Snap pics of what you can't do if you hit a wall, I'll help you. I know how shitty of a job it is...