Start a new world. Size doesn't matter, but if you're in a hurry make a small world.

Progress through the game as you normally would, get decent pre-eye gear, good health/mana, and make sure you have enough housing, because you're going to need the Dryad

The next step is to obtain a suspicious looking eye. You can do this by combining 6 lenses (dropped by Demon Eyes which spawn at night) at a demon altar. You can also find suspicious looking eyes in chests underground. Fight the Eye and kill it. This should attract the Dryad.

You're going to need 5 gold to purchase this from the dryad. (prices may vary depending on the version of the game you're currently playing.)

Now that you have the dirt rod, hover your cursor over a block of dirt and move it into the air. Now that you have the dirt in hand, find a tree and drop the dirt right at the base of the tree. Because the tree and the block of dirt cannot exist in the same space at the same time, the block of dirt will drop as an item that you can pick up.

Congratulations, you now have obtained a block of dirt.