How do I put an aquarium heater?

I just started my tank and I'm not exactly sure how the heater should go. It's supposed to be a fully submersible Top Fin Heater. Anyways, does it go horizontally? Or vertical? Or what? And does it go at the top of the tank, or the bottom? Also, if it's fully submersible, what about the cord that comes out at the top? Can That go under water too? (there's that grey rubber thing, then the plug cord comes right out of it) So how far under water can this fully submersible heater go?