The smeltery is a vital tool for advancing in slimefun, as it is needed for every alloy. However, its directions can be unclear at first. Firstly, its multiblock recipe includes flint and steel, which isn’t a block.
Worry not. All this means is that you need to have a block of fire under the dispenser (which MUST have the hole facing up) like this:
This fire will go out periodically (up to like 8 smelts if RNJesus smiles upon you), so relight it whenever this happens. This necessitates that you have access to the fire block after building the machine.