How do I make it so that upon smelting an item a separte item is placed in slot 0 of the furnace

Use the global trigger item smelted. Check what item is smelted if it’s the one you want then set a variable to true on the player.

The on player tick update, check whether gui furnace is open and then check is variable is true. Then set item in slot 0 to be whatever you want and set variable to false.

You would really need to check whether there is anything in slot 0 already but depends on whatever you’re doing.