You will find Ser on the southern battlements of Skyhold. Speak with him to begin the quest ‘Way of the Templar’. To complete the quest and unlock the Way of the Templar skill tree, you will need to craft a Templar Lyrium Philter. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Templar methods - Found on the top floor of armoury in Skyhold or purchased from the merchant Willvan in Val Royeaux
3x Broken Philter - Obtained from Pride Demons, Rage Demons, and Terror Demons in the Hinterlands regions of Witchwood, West Road, and Lady Shayna’s Valley
10x Embrium - Found throughout the Hinterlands, particularly Crestwood, Exhalted Plains, and Emerald Graves. Also obtained from war table mission ‘Emerald Graves Resources’ by selecting Josephine or Leliana
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.