This contact poi tutorial covers how to do fishtails for poi. I'll cover an exercise that'll give you the basic motor skills and work you up to showing how to do fishtails. For contact poi tricks, you're probably going to want to use poi that has a ball-shaped head made from stage balls or something similar.
Balance the head on the back side of your hand by sticking out your index finger and pinkie and folding down your middle and ring fingers. The tether should be out to the thumb side of your hand.
Swing the tether back and forth to build up momentum and swing it down and away from you with enough force to spin the tether and handle over to the other side of your hand like in a reverse weave. Relax to let the tether fall back into pendulums.
Build up momentum again by swinging the tether and handle and swing them all the way up and over to the side of your hand that you started on.
Once this gets to feel comfortable, try to go back and forth without stopping for a pendulum from either side of the hand. Then go back and forth twice, three times, four times, etc.
Make sure to keep your hand cupped to keep the poi head stable--you'll find that the poi head does a small jump every time you weave to either side. This trick takes a LOT of practice! Don't get discouraged if it doesn't come to you right away.