Dig a 1-block-deep 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
Put a non-flammable block on the sides to contain the liquids
Mine the block where the water and lava meet
Repeat mining the newly formed cobblestone
Add a piston or hopper system if you want automation
Add a roof or walls if you want to keep the generator safe