Smooth Stone is a Cobblestone block that goes through the smelting process twice. Therefore, to get Smooth Stone in Minecraft ...

Arrange the Stone Blocks: In the crafting grid, place four stone blocks in a square formation. Specifically, place one stone ...

Place the stone back into the furnace in the upper slot and it will yield you Smooth Stone with the same 1:1 ratio. The most efficient fuels to use in the furnace are coal and charcoal.