Dig a 4-block-long trench
Place 1 water source at one end
Place 1 lava source at the other end
Leave 1 empty block between the water and lava
Make sure the water flows toward the empty block
Make sure the lava flows toward the empty block
The flowing water and lava will meet and create cobblestone
Mine the cobblestone with a pickaxe
Refill the mined block as needed to keep generating cobblestone