In order to enhance the equipment with jewels, go to the Enhancing tab in the Gemmery.

The higher the type of the equipment, the more slots there for jewels. The first three slots of equipment are used for jewels, the fourth (available only for legendary equipment) for runes. Remember: you cannot enhance a piece of equipment with the same jewels. For example, you cannot inlay two Onyxes in one piece of equipment.

Enhancing takes place instantly.