Decide which side will be the bottom and the top. On the top, cut off a small wedge so the watermelon sits flat. Even if you plan to put it on a stand, this helps it stay stable while you scoop.
Cut the top. You'll want a sharp knife, but not too big. A larger knife actually makes it more difficult to turn and make a nice circle. I used a 6-inch chef's knife. Also make sure to cut the top at an angle so the top will fit back on without falling into the watermelon. Having a top prevents debris and bugs from getting in.
Scoop it out! Get a large bowl to put all the innards in. These flesh will (hopefully) be used for something else, but it won't come out all pretty in slices or cubes. Use a large metal spoon and scoop it out leaving some on the sidewalls. Do this all the way to the bottom. Set the flesh aside and use to make a cocktail or in a salad.
Use the coring tool that came with your metal spigot, an apple corer or a small paring knife to create the hole for the spigot. Make sure to place it low enough that the liquid doesn't need to always be half full in order to flow. This usually means it is at a slight angle and will need to dangle off the table or be on a stand. That is normal.
Insert the spigot. If you are using a metal one, use the rubber stopper on the inside to prevent leaks. Secure in place.
Wait until you are ready to serve to fill with your favorite beverage. Unless, you are just filling it with vodka, then you want it to be able to sit for 3-4 hours to fully infuse.
Keep your watermelon keg chilled until ready to serve. Like any fruit, they get a little soft and funky if out too long, especially in hotter temperatures.
If you've tried this, come back and let us know how it went!
Notes
*I prefer metal, but you can use plastic. See notes above regarding this choice.