How are people managing to make smooth stone?

yeah the furnace twice thing is right. you can skip the first trip if you've got a silk touch pickaxe, mining stone with it drops stone instead of cobble, so it only needs the one smelt. smooth stone slabs are easiest in a stonecutter too, one block in gives you two slabs.

assuming this is minecraft, it's two trips through the furnace. cobblestone smelts into stone, then that stone smelts again into smooth stone. the catch is it has to be a normal furnace. a blast furnace won't take it, which is where a lot of people get stuck.