You will find Your Trainer near the front gate of Skyhold. Speak with her to begin the quest ‘Way of the Rift Mage’. To complete the quest and unlock the Way of the Rift Mage skill tree, you will need to craft a Tome of Rifts. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Rift Magic - Found on the table in the room where Solas can be found adjacent to the throne room, leading to the library. Can also be purchased from the merchant Willvan in Val Royeaux
3x Venatori Tome - Obtained from any Venatori throughout Exalted Plains and the Hinterlands, particularly at Ghilan’nain’s Grove, Halin’sulahn, and Desolate Bank
10x Ring Velvet - Found from Venatori and Freemen of the Dales in Exalted Plains. Can also be obtained from war table mission Gather Cloth by selecting Josephine
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.