In crafting, you must first know the recipe of what you're trying to craft, have the necessary materials in your material tanks for you to extract, and then place the ingredients into the Cauldron in the correct order. If done correctly, you will create a new item that can be sold in your shoppe or given to your champion.
To use the material tanks, walk up to the taps and look at the raw material you wish to extract, then press the left mouse button. Doing so will give you one liter of whatever material you extracted. The liquid material can then be used in the cauldron to craft a new item, or sold in the shoppe for money.
Keep in mind, you can only extract one liter of material at a time - no more / no less. That being said, if you only have 0.9 liters of a raw material, you cannot extract it. However, you can extract liters of any raw material as many times as you want. The material tanks will hold a maximum of 40 liters of each liquid material, and anything beyond this limit is lost.