How do I add a Spirit to my gear?

In the UPGRADE option of each item you can find the Socket option, as below:

To be able to place a Spirit on any gear item first you have to Drill the item. Below image shows Drill option:

Notes:

To create a Socket you need 1 Drill or 50 gems (fix price regardless of item type).

Inserting a spirit costs coins.

Removing a spirit costs gems.

Destroying a spirit is free of charge.