You will find Helaine near the front gate of Skyhold. Speak with her to begin the quest ‘Way of the Knight-Enchanter’. To complete the quest and unlock the Way of the Knight-Enchanter skill tree, you will need to craft a Spirit Blade Hilt. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Knight-Enchanter methods - Found on the table near Vivienne in Skyhold, who is located in the courtyard with Cassandra or on the balcony near Solas. Can also be purchased from the merchant Willvan in Val Royeaux
3x Wisp Essence - Obtained from pure wisps in the Fallow Mire regions of Old Thoroughfare, the Weeping Spires, and Granite Point. Can also drop from wraiths at Fade Rift locations, or from Barnabus in Val Royeaux if The Short List perk is active
10x Lazurite - Found throughout the Hissing Wastes and the Western Approach. Can also be obtained from the war table mission Hissing Wastes Resources by selecting Josephine or Cullen
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.