first, dig down 1 block right there
put fire proof blocks like that. left space, water will be there and lava will be at right. (i am using glowstone just to prevent water from freezing)
pour water at left side (don't put it at top of hole) and lava on right
and then break block between water and lava. it's done! you will see cobble stone forming at middle
while after you mine that cobble stone, soon another cobble stone will formed at the same place. you can get infinite cobble stone.