Start by learning a basic cradle: spread all your fingers out as though you're stretching them away from your palm to create a small bowl on the back side of your hand. You're going to place the poi tether between your index and middle fingers and balance the poi head atop your knuckles.
Practice walking around with your poi in this position to get a feel for keeping it stable--your body is going to want to grip the poi to keep it from falling and that will actually make the fall inevitable. You need to train your body to open rather than close your hand in order to keep the poi from falling.
Now start to swing the poi tether back and forth beneath your hand in a pendulum. When you feel the handle or tether tap against your elbow, you can initiate the contact roll by flicking your fingers upward. The poi will roll to the end of your forearm and you should be able to catch the tether as it comes back around up toward your hand.
The first few times you do this, it's almost certain that the poi will not roll where you want it to. This will be because your body's built-in response to initiate the roll is to tilt your forearm and this will cause you to lose control of the poi.
Instead, train yourself to keep your forearm perfectly level as the poi rolls to keep the poi as much in control as possible. The momentum for the roll comes not from the angle of the path the poi is following but from the movement of the tether. Just so long as the tether swings up and around, there is no need to raise your forearm. This will take some training to get built into your body.