Gather 1 bucket of water
Gather 1 bucket of lava
Gather 1 pickaxe
Dig a 1-block-wide trench
Make the trench at least 4 blocks long
Place a nonflammable block at one end
Place water on one side so it flows toward the center
Place lava on the opposite side so it flows toward the center
Make sure the water and lava meet on a block space
Mine the cobblestone that forms
Repeat mining to keep generating cobblestone