Engineers take advantage of their inventiveness to create an immense (and occasionally random) variety of helpful items. By tinkering ceaselessly, and tolerating malfunctions and misfires, an engineer can make utterly unique objects: sight-enhancing goggles, potent guns, robot pets, mechanical mounts, and even more unusual trinkets. Successful engineers use their inventions to solve problems and make life easier, faster, and better for themselves and their companions.
The majority of Engineering schematics can be manufactured anywhere, provided you've already obtained the diverse materials necessary for their creation.
It's not uncommon for Engineers to also specialize in Mining - the metals and ores they procure serve as building materials for their inventions.As you progress in your studies as an Engineer, you'll be given an opportunity to choose a specialization. After completing a unique quest, you can specialize in either Goblin or Gnomish schools of engineering. Each school specializes in crafting unique items, some of which aren't available to the opposing style of craftsmanship.
Make sure that you have learned the schematic you'd like to use, and that you have the materials listed on that schematic. A number of materials useful for engineering schematics are mined, while others can be purchased from a trade goods vendor or, often, through the auction house.
Once you have the materials you need in your inventory, you can use engineering to craft at any time and nearly any location - simply choose 'engineering' from your professions menu, choose the schematic you'd like, and click 'create'.