Locations of the owls are in the video below (not mine). Make sure to pick up the shards each one drops as they're required for this achievement as well as
Timestamps:
19:00 The Old Watchtowers owl
21:25 Overgrown Ruins owl
24:24 Estate of the Urn Witch owl (via Ceramic Manor door)
AcaelusT notes in the comments that this cannot be done until the postgame.