Develop a nice side-on action
If you can bowl off-spin with a tennis ball, you can restrict opposing batsmen. In order to be a good off-spinner with either a cricket or a tennis ball, you must have a nice action, which allows you to put more revolutions on the ball.
It is always recommended that an off-spinner should bowl with a side-on action, which means that your chest should be pointing towards mid-wicket (leg side), while watching the batsman with your left eye.
A good side-on bowling action allows an off-spinner to turn the ball even on surfaces not conducive to spin.
Ideal grip for an off-spinner
You need to grip the ball between your index finger and the middle-finger, with the thumb playing a supporting role. Your grip should not be too firm, or you will end up pitching the ball short. Similarly, it should not be too loose either or else you will end up bowling full-tosses.
Bowl with a high arm
It is imperative for an off-spinner to bowl with a high-arm, if he wants to extract maximum spin from the wicket. Your arm should be right alongside your right ear when you are about to deliver the ball.
Pivoting the body properly is crucial for a spinner. At the time of delivery, all of your weight should be on the front toe (left toe for a right-arm off-spinner) and should pivot sharply in an anti-clockwise direction. At the time of landing, your front toe should be pointing towards fine-leg and must finish towards third-man after the pivot.
Change of pace
Tennis ball cricket is all about hitting big shots. Therefore, in order to prevent being hit out of the park, you must vary your pace, bounce and length.