Place your strong hand near the back of the ball so that your thumb almost touches the back point. Now place your weak hand near the front of the ball so that your little finger is near the front point. Now when throwing, thrust both your hands forward at equal speed and momentum. This will help you to aim where you want the ball to go and it will also help you to spin the ball. Also to help spin the ball, you should jerk your weak hand up and your strong hand down just as you're about to throw. Words can only explain so much, so like the other guy said, find some videos that show lineouts (overhead throws).
There are some great videos on youtube for rugby. They walk you through different skills. Go to youtube.com and search rugby lineouts. I am sure you will find at least a few videos that will be a lot better than reading an answer.
