To make a smelting recipe, create a file in the 'config/customSmeltingRecipes' folder with an extension of '.srecipe'. Then you have to call for each recipe the smeltingRecipes.addRecipe() function.
With arguments it looks like that:
smeltingRecipes.addRecipe("[input] [output] [count]");
input is the inputid. This can have a damage value (i.e 35-2).
output is the output block/item. This can have a damage value (i.e 35-3).
count is the output count.
smeltingRecipes.addRecipe("4 1 1");
This would be the smelting recipe for cobblestone -> cleanstone.