I decided to abandon my own plans mainly due to time and cost and ordered the parts from SparkFun. What follows is essentially the same information you'll find on their guide but with a few of my own notes. I hope that you find my insights helpful and it will get your project off the ground without a hitch.
Step 1: Parts and Safety
Now a brief note about safety. Every time you work with electrical lines you may be risking your life if you don't use the right precautions. In general you should always employ a certified electrician but you can do this project on your own if you're careful. Absolutely ensure the plug is not connected to a live electrical socket when working on the relay, the outlet, or the extension cord at any point. Also, it's probably good practice to enclose any wires before testing. With that you should probably do just fine.