After "Walking With Spirits" :
Enter the Cathedral.
Talk to Nadine.
Talk to Desmond.
Go find Jackson and his "spiritual guide".
Talk to Desmond... Again.
Go to the amusement park to install the device*.
Make your way back to the mansion.
Enter the saferoom.
Talk to Desmond... Yet again.
Follow Desmond to the lighthouse and kill every robot.
Kill the brain**.
Achievement unlocked !
* : or not... I choosed to help Desmond, I can't tell what happens if you choose to destroy the device.
** : again, or not. Don't know what happens when you kill Desmond.