Dig a 6x1 hole.
Dig one block down on the 3rd block to the left.
Now place down a water source on the far right. The end of the water source should fall down into the hole.
Finally, you place down a lava source on the far left and shortly after, a cobblestone should appear on the 2nd block to the left.
Dig a hole right next to the cobblestone so when you mine the cobblestone, you can safely retrieve cobblestone without having the risk of being burned by the lava.
Congratulations! Now go and enjoy your useful cobblestone generator!
The creator of this guide has not included tools