The Hellfire Forge is used to craft recipes that require Demonic Will and other things (more on that later):
The 4 left outside slots are used for crafting materials, it does not matter where these items go. The 5th middle slot is where your crafted item will appear, the far right side of the forge GUI has a bar that will fill up with Demonic Will when you start to craft. The right-hand slot is your fuel slot (Demonic Will in this case).
When crafting the Demonic Will will not be consumed but it will lose its “Will Quality”. when you first start crafting you might notice that you have all the required ingredients but the Forge will not start the crafting process, this could be because the Demonic Will you are using does not have a high enough “Will Quality”
Use the forge to craft a Petty Tartaric Gem, make sure your Demonic Wil has more than 1 “Will Quality” otherwise your craft will not start.
You can use JEI to find out how much Will is needed to craft certain recipes, you will also notice that some recipes have 2 values:
The minimum is the minimum amount of Will needed to start the craft and Drained is how much Will shall be drained from your gem after the crafting has completed. Think of this as a requirement and a cost, you may require a certain amount of Will but the cost of the craft is different.