They're just a 1 coin cost item at the crafting and refining NPC. You simply need to have them in your bag when crafting.
If an item need say, 5 peach branches and 1 filler, than just buy a filler from the NPC, take it and your branches in your bag, and craft the item, it works exactly like any other crafting material. They stack to 5000 in a single bag slot, so you can easily carry many with you. And they onyl cost 1 coin because WoKF code does not allow an NPC so tell an item for 0 coins.
Also, if you have a question for me there is a contact form on the website that you can fill in and it sends the details directly to my email, you can use it for any support, account, in-game, technical etc.