How to make a smeltery

Well, it really depends on how specifically you want it to work. The basic idea would be something like this:

• Make a custom GUI with two input slots and one output slot.
• On the GUI's update tick trigger, check the items in the two input slots. If they are the components for a recipe, set the resulting item in the output slot. If not, set air in the output slot.
• Whenever an item is taken from the output slot, remove one item from each input slot. (And possibly play a cool sound effect.)

If you want it to have a progress bar, (like the furnace), that's a little bit trickier, but not impossible. (The basic idea is that you give your block an NBT number tag that counts up, consuming the fuel and ingredients to make the product once it reaches a certain point, and use some additional image overlays for the progress bar.)