Gather 1 bucket of water
Gather 1 bucket of lava
Gather building blocks
Dig or build a 1-block-wide trench
Place 2 blocks with a 1-block gap between them
Put water on one side of the gap
Put lava on the other side of the gap
Let the water and lava meet to form cobblestone
Mine the cobblestone with a pickaxe
Refill the mined block to keep generating cobblestone