From the Leatherworking FAQ on the official forums:
http://forums.worldofwarcraft.com/thread.html?topicId=11871188&sid=1
3.3 How do I make something?
- Find your Leatherworking icon on your abilities page and click it to bring up the Leatherworking UI.
- Click on the item you'd like to make to see the required components.
- If you have the required components in your inventory, click on the 'Create' button.
- If you want to make many of the same item at once, you can change the number at the bottom of the window or click the 'Create All' button to make as many as you have the components for.