Many people believe it takes a mastermind to completely control their partner, but this is not really necessary. An ability to learn their partner's fears and hot button issues are generally all they need to get started. After that, they plant many seeds of doubt and distrust into their partner's life. They might discuss an issue with their partner and mention how family and friends will respond incorrectly to them. They then wait, knowing the issue will be brought up by friends and relatives. This will set the distrust in motion as their partner finds friends and family react as negatively as their controlling partner predicted.
It does take time to establish full control of a partner and ensure their isolation. Emotional abuse is part of the control, and this is often done by being negative about any facet of their partner's interactions with other people. If a partner likes a friend's new haircut, the controller will point out, in private, how embarrassing it was to hear the person never liked their friend's hair. Small steps like these are how the isolation and total control begins.