To make this "mud battery" you'll need:
An Empty Container
Insulated Copper Wire
At Least 5 cm of Graphite (taken from a pencil)
A Digital Multimeter
A Hand Shovel
Step 1: Obtain and Insert the Dirt
Besides finding the right kind of soil to use, this step really is easy as just filling up your container with dirt.
Step 2: Prepare the Anode
Step 3: Prepare the Cathode
Step 4: Turn the Dirt Into Mud and Then Wait
Now feel free to correct me if I'm wrong on this, but I've found that even though the battery does (or at least should) generate an electric potential, wiring several of these devices together in series in an attempt to get at least a little function from them provides a far lower voltage than would be expected due to (what I'm assuming is) the high resistance posed by each battery in the series.
Though I knew from I've found, however, that moistening the dirt until it becomes mud greatly ameliorates this situation, as mud cells (in comparison to dirt cells) not only boast higher voltages (0.8-1.0 volts) but also seem to fair far better when wired into a series.
This step is pretty much as easy as it sounds: just add water (tap water works) to your mud battery until the soil within it is completely saturated with water.
The benefits of mud over dirt in this battery aren't immediately presented; to see the increased voltage one must allow the battery to incubate overnight. While I don't really know exactly why it has to incubate over night to work (I initially thought that it was because the voltage was generated by microbial activity and that the increase in voltage following incubation was due to microbial colonization of the anode), if I had to guess I'd say that the incubation is likely necessary because it takes time for the water to dissolve some of the salts within the soil into solution.
People to Thank
In closing I'd kind of like to thank egbertfitzwilly because I gleaned a lot of what I know of homemade electrodes from reading his instructables and HarveyH44 and Kiteman because they pointed out why what I thought I made (a microbial fuel cell) wasn't what I actually made and showed me what it was I actually concocted (an Earth battery).