You need to purchase a Leatherworker's Tool Belt from the trainer.
Skill Mechanics Edit
To begin creating items, right-click on the Leatherworker's Tool Belt in your inventory.
Increasing the skillEdit
To increase your skill, you must produce items, that would grant you the highest amount of profession points. For this to work you must either, have a secondary profession of Disassembling or you must purchase items that are needed for you to create something successfully, such as Damaged Leather Patch from the Auction House, which is located in Yasker's Tower.