The details are in the guide but a step by step one... You need to finish the find the Culprit quest. This quest is given to you by Keppler in the library after day 3. Then you can go to the classroom across potions, check the desk with the red doodles. Then submit quest. You can also go confront Winston in the sparring field after reading his doodles, but not after you reported him.
After that is done, when later Thane loses it, he will ask you to bond with him. Don't cure him, or anything like that, just bond with him when he asks after Io's arc. To be honest that's it. If you've bonded with him, he will be your lover, and follow you to your world