You can use it exactly like a normal furnace, just put your items in, add fuel and let it smelt. The difference is that some desert related items are 2x faster when using the Sandstone Furnace. Currently Sand, Red Sand, Cactus, Sandstone, Red Sandstone and Terracotta are part of the DESERT_SMELTABLE tag and will smelt twice as fast.
Every other item still smelts at the normal furnace speed, so you dont get some random speed boost for everything.
