Step 1: Dig
Dig a 6x1 block hole
Step 2: Dig More
Dig a little hole in the third block from the left
Step 3: Add Water
Add one tap of water to the very right of the hope
Step 4: Add Lava
Add one tap of lava to the very left of the hole. Once this is done the two liquids will clash and make a cobblestone brick. Break the brick and another one will appear.
Step 7: Finished
Now you have made a cobblestone generator. HAVE FUN!