first craft and place the furnace
second put fuel (wood) in the furnace by pressing Q at the hole on the bottom of the furnace while holding the item
thirdly after placing the fuel put in the item you want to smelt (iron,copper)wait for like 10 seconds or so
and done
ps 1 sometime when you place the item it will fail and fell through the ground
2 when something is smelting there will be smoke coming out of the furnace
3 the ingots will fall out of the behind of the furnace
sorry english is not my main laugauge and does anyone know how to get silicon