The Dominate skill, the Retrain Powers and Advanced Training upgrades, and plenty of Element Zero are required. You can get the Dominate Skill by recruiting Morinth (Samara's daughter), which means you must sacrifice Samara in her own loyalty mission. A list of planets with Element Zero can be found below. First, buy Advanced Training and choose Dominate. This will land you a bonus skill point, which you can use by instituting the Retrain Powers upgrade. Repeat as desired.